WHO IS ALLAH?
We hear about Allah سبحانه و تعالى from different people defining differently based on their understandings. Based on their perception we fix our perception of Allah سبحانه و تعالى. Let us understand about Allah سبحانه و تعالى (the creator) by the best way. And the best way of understanding or knowing about Allah سبحانه و تعالى is by Allah سبحانه و تعالى Himself in the Qur’an, Prophet Muhammad ﷺ and as understood by the male and female Companions of Muhammad ﷺ.
Allah, the Supreme Being of this universe – revealed the message of truth through the Qur’an to enable us to live meaningful lives and to help us succeed in the hereafter. Reflecting on that message with an open mind and heart has allowed many from mankind to recognize and believe in Allah as the sole deity of this universe. Once our minds clearly know Allah as our creator and sustainer, we can better understand Allah’s rights over us and can also help us to worship Him the way we ought to worship Him as our creator. 
Unfortunately there are some People that wish to propagate that this word ALLAH means other than the One God of the Prophets, as if the word refers to some idol or moon god. In Arabic the generic word for deity and god is “ilah” (إله) with the dual as Ilahain (إلهين) and the plural as Aalihah (آلهة). The word Ilah in Arabic means “that which is worshiped” whether rightfully or without knowledge, thus meaning respectively the One True God, or god as in false gods. In the plural of Arabic Aalihah (آلهة) is always for the false gods and idols. To give some examples of usage we can say, for instance for the One True God, in constructions with pronouns like my God “Ilahi,” and your God “Ilahukum,” and with proper nouns like the God of Abraham “Ilahi Ibraheem.” In the following ayath we find “God of your fathers” as Ilaha aba`ika and “One God” as “Ilahan Wahidan” (إلها واحدا)
أَمْ كُنتُمْ شُهَدَاء إِذْ حَضَرَ يَعْقُوبَ الْمَوْتُ إِذْ قَالَ لِبَنِيهِ مَا تَعْبُدُونَ مِن بَعْدِي قَالُواْ نَعْبُدُ إِلَـهَكَ وَإِلَـهَ آبَائِكَ إِبْرَاهِيمَ وَإِسْمَاعِيلَ وَإِسْحَاقَ إِلَـهًا وَاحِدًا وَنَحْنُ لَهُ مُسْلِمُونَ
“Were you witnesses when death approached Jacob and he said to his sons, ‘What will you worship after me?’ They said, ‘We will worship your God and the God of your fathers, Abraham and Ishmael and Isaac, one God. And we are Muslims (in submission) to Him.” Qur’an.Surah Baqrah 2:133
Allah relates the amazement of the Arab polytheists at the claim of monotheism by the Messenger Muhammad, and what they said to him:
أَجَعَلَ الْآلِهَةَ إِلَهًا وَاحِدًا إِنَّ هَذَا لَشَيْءٌ عُجَابٌ
“ And they wonder that there has come to them a warner from among themselves. And the disbelievers say, "This is a magician and a liar. Has he made the gods [only] one God? Indeed, this is a curious thing. .” Qur’an.Surah Sa’ad 38: 4-5
The word “Allah” is the proper name of The One God. Literally it is made up of the letters “al” which is the definite article “the,” and “ilah” the word for god or God (according to context as above). Thus the word “Allah” is “al-ilah” and literally “The God,” as in the Arabic phonetic construction (ال + إله=اللَّهُ). Of course the meaning is the One and Only God and the One Lord and Creator and Sovereign of the creation.
According to Ibn Taymiyya رحمه الله, the name Allah (الله), which has the same Arabic root (‘-l-h) as ulūhiyyah, is associated with worship. Thus in Arabic, the word "Allah" means "The One who deserves all worship". This, in a nutshell, is the Pure Monotheistic message of Islam. Majmūʿ al-Fatāwa 2:456 
The Oneness of Allah سبحانه و تعالى, known as Tawhid or Tawheed, is the first and principal constituent of the Islamic concept, as it is the fundamental truth of the Islamic faith. It is also one of the chief characteristics of the Islamic concept because, among all the belief systems and philosophies currently prevailing among human beings, only the Islamic faith can be characterized as having a pure form of monotheism with clarity of faith.
Allah - there is no deity except Him, the Ever-Living, the Sustainer of [all] existence. Qur’an.SurahBaqarah 2:255
Allah سبحانه و تعالى - there is no deity except Him, the Ever-Living, the Sustainer of existence. Qur’an.Surah Ale Imran 3:2
Indeed, I am Allah سبحانه و تعالى. There is no deity except Me, so worship Me and establish prayer for My remembrance. Qur’an.Surah Ta Ha 20:14
Had there been within the heavens and earth gods besides Allah سبحانه و تعالى, they both would have been ruined. So exalted is Allah سبحانه و تعالى, Lord of the Throne, above what they describe. Qur’an.Surah Anbiya 21:22
"There is no deity except Me, so worship Me." Qur’an.Surah Anbiya 21:25
Say, "He is Allah سبحانه و تعالى, [who is] One. Qur’an.Surah Ikhlas 112:1
Allah سبحانه و تعالى is the Creator of all things Qur’an.Surah Zumar 39:62
Such is Allah سبحانه و تعالى, your Sustainer. There is no god but He, the Creator of everything. Worship, then, Him alone, for He has everything in His care. Qur’an.Surah An 'am 6: 102.
Allah سبحانه و تعالى has not taken any son, nor has there ever been with Him any deity. [If there had been], then each deity would have taken what it created, and some of them would have sought to overcome others. Exalted is Allah سبحانه و تعالى above what they describe [concerning Him]. Qur’an.Surah Muminun 23:91
He to whom belongs the dominion of the heavens and the earth and who has not taken a son and has not had a partner in dominion and has created each thing and determined it with [precise] determination. Qur’an.Surah Furqan 25:2.
To Allah سبحانه و تعالى belongs the dominion of the heavens and the earth, and all that is between them. Qur’an.Surah Ma'idah 5: 17.
He is Allah سبحانه و تعالى, other than whom there is no deity Qur’an.Surah Hashr 59:23
O mankind, remember the favor of Allah سبحانه و تعالى upon you. Is there any creator other than Allah سبحانه و تعالى who provides for you from the heaven and earth? There is no deity except Him, so how are you deluded? Qur’an.Surah Fatir 35:3
And how many a creature carries not its [own] provision. Allah سبحانه و تعالى provides for it and for you. And He is the Hearing, the Knowing. Qur’an.Surah Ankabut 29:60
Say, "Have you considered: if Allah سبحانه و تعالى should take away your hearing and your sight and set a seal upon your hearts, which deity other than Allah سبحانه و تعالى could bring them [back] to you?" Look how we diversify the verses; then they [still] turn away. Qur’an.Surah Anam 6:46
And to Him belongs whoever is in the heavens and earth. All are to Him devoutly obedient. Qur’an.Surah Rum 30:26
They have taken their scholars and monks as lords besides Allah سبحانه و تعالى, and [also] the Messiah, the son of Mary. And they were not commanded except to worship one God; there is no deity except Him. Qur’an.Surah Tawbah 9:31 
Allah سبحانه و تعالى says in the first ayath(or statement) of Qur’an that Allah سبحانه و تعالى is the Rabb of the worlds. And the Arabic word “Rabb” means Creator, Owner and the One, Who rectifies and nurtures them by sending His Messengers, and revealing His Books, and rewarding them for their righteous deeds. Al- Ibn ul-Qayyim (rahimahullah) said, 'Verily, Rububiyah includes commanding the slaves and prohibiting them, and rewarding the good doers with good and reprimand (the evil doers) with punishment.' Madarijus-Salikeen (1/8)
Few references from Qur’an
He is the Provider of all animals, humans and everything else, 'And no (moving) living creature is there on earth but its provision is due from Allah سبحانه و تعالى.' Qur’an.Surah Hud 11: 6
He is the King of the dominion and the administrator of all affairs; He changes the night into day, gives life and death. You bring the living out of the dead, and You bring the dead out of the living. And You give wealth and sustenance to whom You will, without limit.' Qur’an.Surah A’le -Imran 3: 26-27
Allah سبحانه و تعالى has negated any partner or assistant in His dominion, just like He negated any partner in Creation and Provision. Allah سبحانه و تعالى says, 'This is the creation of Allah سبحانه و تعالى. So show Me that which those (whom you worship), besides Him have created.' Qur’an.Surah Luqman 31: 11
Allah سبحانه و تعالى has created all His creation with a natural instinct (i.e. Fitra) that affirms His Rububiyah - including the Mushrikeen (polytheists), who (even though they) ascribed partners with Him in worship, declared His Sovereignty in Rububiyah, as He said, 'Say, 'Who is (the) Lord of the seven heavens, and (the) Lord of the Great Throne?' They will say, 'Allah سبحانه و تعالى,' say, 'Will you not then fear Allah سبحانه و تعالى. Say, 'In Whose Hand is the sovereignty of everything (i.e. treasures of each and everything)? And He protects (all), while against Whom there is no protector, if you know. They will say, '(All that belongs) to Allah سبحانه و تعالى.' Say, 'How then are you deceived and turn away from the truth?' Qur’an.Surah -Muminun23:86-89
In Qur’an.Surah 27:60-64, Allah سبحانه و تعالى said, He alone:
Allah is unique in His names. The names of Allah سبحانه و تعالى are not limited to any specific number. We know some of them and do not know others, as is indicated by the hadeeth of ‘Abd-Allah ibn Mas’ood (رضي الله عنهم), who said: “The Messenger of Allah ﷺsaid: ‘There is no one among you who, when he is stricken with distress and grief, says, O Allah, I am Your slave…..I ask You by every name belonging to You which You named Yourself with, or You taught to any of Your creation, or revealed in Your Book, or You have preserved in the knowledge of the Unseen with You.. Musnad Ahmad, 3582;
The names of Allah سبحانه و تعالى are all husnaa [Most Beautiful], as He says: “And (all) the Most Beautiful Names belong to Allah سبحانه و تعالى, so call on Him by them, and leave the company of those who belief or deny (or utter impious speech against) His Names. They will be requited for what they used to do.” Qur’an.Surah A’raaf 7:180
The name Allah سبحانه و تعالى refers to the One Who is adored and worshipped, the One Who possesses the attributes of divinity and to whom all His creation is enslaved. The attributes of divinity with which He describes Himself are the attributes of perfection. 
Few names of Allah سبحانه و تعالى are:
For more Names of Allah سبحانه و تعالى, refer to NAMES AND ATTRIBUTES OF ALLAH
ALLAH سبحانه و تعالى is the true owner & sovereign of everything in universe, Qur’an.Surah Hashr 59:23
To Him belongs that before us and that behind us and what is in between. And never is your Lord forgetful. Qur’an.Surah Maryam 19:64
ONLY ALLAH سبحانه و تعالى is the director of affairs & regulats order inthe universe Qur’an.Surah Rad 13:2
“Say: ‘Do ye then take (for worship) protectors other than Him, such as have no power either for good or for harm to themselves?” Qur’an.Surah Rad 13:16
His command is only when He intends a thing that He says to it, "Be," and it is. Qur’an.Surah Ya seen 36:82
And you do not will except that Allah سبحانه و تعالى wills. Indeed, Allah سبحانه و تعالى is ever Knowing and Wise.Qur’an.Surah Insan 76:30
Only ALLAH سبحانه و تعالى has the power to declare things as lawful on unlawul...because he himself is the maker of law ..Qur’an.Surah Tahreem 66:1
Only ALLAH سبحانه و تعالى has the knowldege of the unseen Qur’an.Surah Araf 7:188
ALLAH سبحانه و تعالى alone bestows progeny (sons and daughters) ... Qur’an.Surah Ash Shura 42: 49 50
ALLAH سبحانه و تعالى alone who guides, feeds, cures diseases and bestows health, who gives life and death, forgive sins Qur’an.Surah Shuara 26: 78-82
Allah سبحانه و تعالى alone provides provision and success Qur’an.Surah Hud 11:88
ALLAH سبحانه و تعالى alone can grant profit or loss...and he alone is the master of destiny Qur’an.Surah Fath 48:11
Meaning of Ibaadah
In other words, Ibaadah can be defined as a state, and it can be also defined by the actions. Anything that Allahسبحانه و تعالى commands us to do and he is pleased that we do it is an act of worship.
The greatest of all sins is to commit shirk with Allahسبحانه و تعالى , and to feel safe from the punishment of Allah سبحانه و تعالى, and to give up hope of the mercy of Allah سبحانه و تعالى and to despair of Allah’s سبحانه و تعالى aide. 
Few references from Qur’an
It is You (ALLAH سبحانه و تعالى) we worship and You we ask for help. Qur’an.Surah Fatiha 1:4
So whoever would hope for the meeting with his Lord - let him do righteous work and not associate in the worship of his Lord anyone. Qur’an.Surah 18:110
And your Lord says, "Call upon Me; I will respond to you." Indeed, those who disdain My worship will enter Hell [rendered] contemptible. Qur’an.Surah 40:60
And whoever invokes besides Allahسبحانه و تعالى another deity for which he has no proof - then his account is only with his Lord. Indeed, the disbelievers will not succeed. Qur’an.Surah 23:117
And when My servants ask you, [O Muhammad], concerning Me - indeed I am near. I respond to the invocation of the supplicant when he calls upon Me. So let them respond to Me [by obedience] and believe in Me that they may be [rightly] guided. Qur’an.Surah 2:186
And when My servants ask you, [O Muhammad], concerning Me - indeed I am near. I respond to the invocation of the supplicant when he calls upon Me. So let them respond to Me [by obedience] and believe in Me that they may be [rightly] guided. Qur’an.Surah 27:62 
Evidence that only Allah is the Creator of life
Do they not look into the realm of the heavens and the earth and everything that Allahسبحانه و تعالى has created and [think] that perhaps their appointed time has come near? So in what statement hereafter will they believe? Qur’an.Surah Araf 7:185
Then is He who creates like one who does not create? So will you not be reminded? Qur’an.Surah Nahl 16:17
O people, an example is presented, so listen to it. Indeed, those you invoke besides Allah سبحانه و تعالى will never create [as much as] a fly, even if they gathered together for that purpose. And if the fly should steal away from them a [tiny] thing, they could not recover it from him. Weak are the pursuer and pursued.Qur’an.SurahHajj 22:73
But they have taken besides Him gods which create nothing, while they are created, and possess not for themselves any harm or benefit and possess not [power to cause] death or life or resurrection.Qur’an.SurahFurqan 25:3
[He] who created death and life to test you [as to] which of you is best in deed Qur’an.Surah 67:2
Imam al-Bukhari reported in his Saheeh from Abu Sa‘eed (رضي الله عنهم) that the Prophet Muhammad ﷺ said: "There is no created being but Allah سبحانه و تعالى created it." In Arabic, the word "khalaqa" means to make out of nothing, which is something that only Allah سبحانه و تعالى can do; it is impossible for anyone except Allah سبحانه و تعالى to do this. It also carries the meaning of decreeing or foreordaining. Fath al-Bari Sharh Saheeh al-Bukhari, 13/390 
Ahlus-Sunnah wal-Jama'ah believe in and confirm all of the attributes of Allah without distorting their meaning, and that Allah is above His seven heavens, above His 'Arsh, and separated from His creatures, and His creatures are separated from Him.
Indeed, your Lord is Allah سبحانه و تعالى, who created the heavens and the earth in six days and then established Himself above (Istawa) the Throne (really in a manner that suits His Majesty).Qur’an.Surah Yunus 10:3.
The Most Gracious (Allah) rose over (Istawa) the (Mighty) Throne (in a manner that suits His Majesty). Qur’an.Surah Taha 20:5.
The Angels ascend up to Him. Qur’an.Surah Maarij 70:4.
Every good saying goes up to him. Quran.Surah Fatir 35:10.
The revelation brought down from your Lord. Qur’an.Surah Nahl 16:102.
Lord is above them who fear. Qur’an.Surah Nahl 16:50.
And there are many more proofs from Qur’an which makes it clear that Allah سبحانه و تعالى is above His creation.
"Nothing ascends to Allah سبحانه و تعالى except good." Sahih Al Bukhari vol 9:525; Sahih Al Muslim 1320. “.
Allah سبحانه و تعالى wrote a book which is above the throne with him...” Sahih Al Bukhari vol 9:518.
Zaynab رضي الله عنه: says “Allah سبحانه و تعالى from above the seven heavens gave her away in Marriage” Sahih Al Bukhari vol 9 : 517.
Narrated Abdullah ibn Amr ibn al-'As رضي الله عنه: The Prophet Muhammad ﷺ said: The Compassionate One has mercy on those who are merciful. If you show mercy to those who are on the earth, He Who is in the heaven will show mercy to you. Abu Dawud 4941 and Tirmidhi 1924
Narrated Ibn Abbas رضي الله عنه: When the Prophet Muhammad ﷺ has sent Muadh رضي الله عنه to Yemen, he said to him, "You are going to a nation from the people of the Scripture, so let the first thing to which you will invite them, be the Tawheed of Allah. If they learn that, tell them that Allah سبحانه و تعالى has enjoined on them, five prayers to be offered in one day and one night. And if they pray, tell them that Allah سبحانه و تعالى has enjoined on them Zakat of their properties and it is to be taken from the rich among them and given to the poor. And if they agree to that, then take from them Zakat but avoid the best property of the people." Sahih Al Bhukari Vol 9: 469
Narrated Mu'adh bin Jabal رضي الله عنه: The Prophet Muhammad ﷺsaid, "O Mu'adh! Do you know what Allah's Right upon His slaves is?" I said, "Allah سبحانه و تعالى and His Apostle know best." The Prophet Muhammad ﷺ said, "To worship Him (Allah) Alone and to join none in worship with Him (Allah). Do you know what their right upon Him is?" I replied, "Allah سبحانه و تعالى and His Apostle know best." The Prophet Muhammad ﷺsaid, "Not to punish them (if they do so)." Sahih Al Bhukari Vol 9: 470  
Allah سبحانه و تعالى is the most merciful. He does not want us to enter hell fire. So Allah سبحانه و تعالى extends His mercy so much that He multiplies our deeds several times and forgives us countless times.
Indeed, Allah سبحانه و تعالى does not do injustice, [even] as much as an atom's weight; while if there is a good deed, He multiplies it and gives from Him a great reward. Qur’an.Surah Nisa 4: 40
Whoever comes [on the Day of Judgment] with a good deed will have ten times the like thereof [to his credit], and whoever comes with an evil deed will not be recompensed except the like thereof; and they will not be wronged. Qur’an.Surah Anam 6: 160
Prophet Muhammad ﷺ said: It is narrated on the authority of Ibn Abbas رضيالله عنه that the Messenger of Allah ﷺ transmitted it from the Blessed and Great Lord: “Verily Allah سبحانه و تعالى recorded the good and the evil and then made it clear that he who intended good but did not do it, Allah سبحانه و تعالى recorded one complete good in his favour, but if he intended it and also did it, the Glorious and Great Allah سبحانه و تعالى recorded ten to seven hundred virtues and even more to his credit. But it he intended evil, but did not commit it, Allah سبحانه و تعالى wrote down full one good in his favour. If he intended that and also committed it, Allah سبحانه و تعالى made an entry of one evil against him.” Sahih Muslim 237
Why would Allah سبحانه و تعالى go out of His way to multiply good deeds, record good deeds when you have not done it, record good deeds when you had intended evil and record just 1 evil deed for every evil action? There can be no other reason except that Allah سبحانه و تعالى is most merciful and does not want us to go to Hell.
You will realize how incorrect it is to estimate Allah سبحانه و تعالى like a mathematics teacher or a business man. One can appreciate that Allah's mercy has no boundaries. Alhamdulillah!
Is THE GOD "Allah" only for Islam and Muslims?
"Allah" is the same word used by Christian and Jewish Arabs in the Bible to refer the one and only God ALLAH.
The Arabic Bible beginning with the Book of Genesis uses the word "Allah" to represent "The God" of Creation; Adam and Eve(Aadam and Hawwa); Noah(Nuh); Abraham(Ibraheem); Ishmael(Is’maeel) and Isaac(Ishaq) and Israel(Yakub)[Peace be upon them All]. On page number one  and Chapter one of Genesis in the Old Testament of the BIBLE, Arabic Version, we find the word "Allah" seventeen  times. 
DOWNLOAD the word page from the link – http://www.arabicbible.com/arabic-bible.html
For more on this topic refer to:The book ‘Sharh Hadeeth Jibra’eel’ by Shaikh ibn Saleh al-Uthaimeen (with additional quotes from Shaikh Salih ibn Fawzan ibn Abdullah al-Fawzan) highlight and summarize the various aspects of Tawheed that are held by Ahlus-Sunnah (Followers of Sunnah).