The word 'Abd' means the subdued or submission. This meaning encompasses all the types of creation, higher and lower, rational and reasonless, animate and inanimate, dynamic and static, apparent and hidden, believing and disbelieving, and pious and sinning etc.. All of them are the creation of Allaah, the Mighty and Majestic. They are subjected to Him, controlled and managed by Him alone. The Arabic word ‘Abd ’ is translated in english as slave.
And if the intended sense is the worshipper, the one who loves Allaah, the one who is humble, then it refers only to the believing men and women who are Allaah's honourable slaves and those who are close to Him, who fear Him and have piety, they the ones who shall not fear, nor shall they grieve. 
All of them have a limit and destination and each one runs (its course) for an appointed term and does not exceed that term by an atoms worth, “Such is the judgment of the Mighty, the all-Knowing” Qur’an Surah Yasin36: 38
And when My slaves 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 SurahBaqrah 2:186
And of the people is he who sells himself, seeking means to the approval of Allah. And Allah is kind to [His] slaves. Qur’an Surah Baqrah 2:207
So if they argue with you, say, "I have submitted myself to Allah [in Islam], and [so have] those who follow me." And say to those who were given the Scripture and [to] the unlearned, "Have you submitted yourselves?" And if they submit [in Islam], they are rightly guided; but if they turn away - then upon you is only the [duty of] notification. And Allah is Seeing of [His] slaves Qur’an Surah A’le Imran 3:20
Musa (Moses) said to his people: "Seek help in Allah and be patient. Verily, the earth is Allah's. He gives it as a heritage to whom He will of His slaves, and the (blessed) end is for the Muttaqun Qur’an Surah Araf 7:128
Verily, those whom you call upon besides Allah are slaves like you. So call upon them and let them answer you if you are truthful. Qur’an Surah Araf 7:194
Declare (O Muhammad SAW) unto My slaves, that truly, I am the Oft-Forgiving, the Most-Merciful. Qur’an Surah Hijr 15:49
Say: "Sufficient is Allah for a witness between me and you. Verily! He is the All-Knower, the All-Seer of His slaves."Qur’an SurahIsra 17:96 
The Prophet (peace and blessings of Allaah be upon him) said: "When Allaah wills good for His slave, He hastens his punishment in this world, and when He wills bad for His slave, He withholds his sins until he comes with them on the Day of Resurrection." Sunan al-Tirmidhi, 2396; classed as saheeh by al-Albaani
Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “Allah has one hundred parts of mercy, of which He sent down one between the jinn, mankind, the animals and the insects, by means of which they are compassionate and merciful to one another, and by means of which wild animals are kind to their offspring. And Allah has kept back ninety-nine parts of mercy with which to be merciful to His slaves of the Day of Resurrection.” Sahih Muslim, al-Tawbah, 6908
It was narrated that Abu Hurayrah(R) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: "Whosoever shows enmity to someone devoted to Me, I shall be at war with him. My slave draws not near to Me with anything more loved by Me than the religious duties I have enjoined upon him, and My slave continues to draw near to Me with supererogatory works so that I shall love him. When I love him I am his hearing with which he hears, his seeing with which he sees, his hand with which he strikes and his foot with which he walks. Were he to ask [something] of Me, I would surely give it to him, and were he to ask Me for refuge, I would surely grant him it. I do not hesitate about anything as much as I hesitate about [seizing] the soul of My faithful servant: he hates death and I hate hurting him." Sahih al-Bukhari, 6502
This hadeeth qudsi includes a number of benefits of Allaah’s love for His slave:
1. "I am his hearing with which he hears" i.e., he does not listen to anything but that which Allaah loves.
2. "his seeing with which he sees" i.e., he does not see anything but that which Allaah loves.
3. "his hand with which he strikes" i.e., he does not do anything with his hand but that which Allaah loves.
4. "and his foot with which he walks" i.e., he does not go towards anything but that which Allaah loves.
5. "Were he to ask [something] of Me, I would surely give it to him" i.e., his du’aa’s (supplications) are heard and his requests are granted.
6. "And were he to ask Me for refuge, I would surely grant him it" i.e., he is protected by Allaah from all things… 
The first obligation upon the slaves is to have recognition of the purpose for which Allaah created them; and for which He took a covenant from them and the purpose for which He sent them His Messengers and revealed His Books; and for which He created this present life, the Hereafter, Paradise and Hell-fire; and for which the Inevitable would come true; and the Event would befall; and for which the Scales are set up and the Records fly; and for which there would be either happiness or misery; and according to which the lights would be divided: “And whoever Allaah gives not light, there will be no light for him” (an-Noor: 40)