MARRIAGE IN ISLAM
Islam encourages the young Muslims to get married and have children. The Prophet Muhammad (May Allah honor Him and grant Him peace) said, 'O young people, whoever of you can afford it, let him get married. It helps restrain the eyes and preserve the private parts. But if he cannot afford it, let him fast, for it is a means of controlling the sexual desire.' Sahih Al Bukhari Vol 7: 4 or 5065(NE), Sahih Al Muslim 3233. 
The state of being united to a person of the opposite sex as husband or wife in a consensual and contractual relationship recognized by law. 
Nikah is an Arabic term used for marriage. It means "contract" ("aqd in Arabic).
The Quran specifically refers to marriage as "mithaqun ghalithun," which means "a strong covenant".
"...and they have taken a strong pledge (mithaqun ghalithun) from you?" (Quran 4:21)
The seriousness of this covenant becomes obvious when one finds the same tern i.e., Mithaqun Ghalithun, being used for the covenant made between Allah and the Prophet before granting them the responsibility of the Prophethood. (Quran 33:7)
The order to Marry is given as Narrated by Anas bin Malik® : A group of three men came to the houses of the wives of the Prophet asking how the Prophet worshipped (Allah), and when they were informed about that, they considered their worship insufficient and said, "Where are we from the Prophet as his past and future sins have been forgiven." Then one of them said, "I will offer the prayer throughout the night forever." The other said, "I will fast throughout the year and will not break my fast." The third said, "I will keep away from the women and will not marry forever." Allah's Apostle came to them and said, "Are you the same people who said so-and-so? By Allah, I am more submissive to Allah and more afraid of Him than you; yet I fast and break my fast, I do sleep and I also marry women. So he who does not follow my tradition in religion, is not from me (not one of my followers)." Sahih Al Bukhari Vol 7: 1. 
Choosing the Right Spouse
These are the main characteristics that should be sought when choosing a spouse:
Deen (way of Life as prescribed by Allah): It is the first thing to look for according to The Prophet Muhammad (May Allah honor Him and grant Him peace) instructions. This does not mean choosing any Muslim but the one who applies Islam throughout his daily life, not just a nominal Muslim.
"A woman may be married for four reasons: for her property, for her rank, for her beauty, and for her religion (and character), Prophet Muhammad (May Allah honor Him and grant Him peace) assured the bounty of Allah to those who wish to get married and live a pure and clean life.
Conduct: A woman or a man who has good manners is considered as a grace from Allah (Glory be to Him).. The Prophet Muhammad (May Allah honor Him and grant Him peace) said, 'There is nothing more beneficial to a believer next to the fear of Allah (Glory be to Him). than a pious wife. When he commands her, she obeys him; and when he looks at her, she pleases him; and when he swears by Allah (Glory be to Him) that she should do a thing, she would; and when he is away, she guards herself and his property.' Musnad Ahmad Vol 2:251; classed as hasan by al-Albaani in al-Silsilah al-Saheehah 1838.
This also applies to the husband with regard to good conduct and fear of Allah (Glory be to Him), for raising children requires the efforts of both parents not just one of them. Since man is the head of the family, it is his responsibility to choose a pious woman to be the mother of his children. Allah (Glory be to Him). states, 'O you who believe, protect yourselves and your families from a fire whose fuel is man and stones.' Qur’an Surah Tahrim 66:6
When The Messenger of Allah (May Allah honor Him and grant Him peace) was asked about the greatest sins, he replied, 'You should associate an equal to Allah (Glory be to Him) while He created you, that you kill your child out of fear that he would eat from your food, and that you commit adultery with your neighbour's wife.' Sahih Al Muslim 157 and 156.
The prohibition of killing one's children is a necessary consequence of having mercy for them and protecting their body, minds and soul from harm. 
Islam gives utmost importance to marriages. Marriage is the most basic remedy for sexual appetite.
1. It is obedience to the command of Allah (Glory be to Him). Allah (Glory be to Him)., who says in the Qur’an: “Marry those among you who are unmarried and your slaves and slave girls who are righteous. If they are poor Allah will enrich them from His overflowing favor.” Qur’an.Surah Noor 24:32.
2. It is following the Prophet Muhammad (May Allah honor Him and grant Him peace). Allah (Glory be to Him). says in the Qur’an “We sent Messengers before you and gave them wives and children too” Qur’an.Surah Raad 13:38.
3. It refreshes the mind, delights the soul and eases off tensions. The Almighty says: “Among His signs is that He created for you spouses from yourselves so that you might find repose with them. And He has placed between you affection and mercy. In that there are certainly signs for people who reflect.” Qur’an.Surah Room 30:21.
4. It fortifies the heart against Satan, it helps lowering the gaze and satisfies the soul. 
Islam has given freedom of choice to those who wish to get married. The mutual choice of the would-be-spouses is given the highest consideration: "Do not prevent them from marrying their husbands when they agree between themselves in a lawful manner." Qur’an.Surah Baqrah 2:232.
Under the Shari'ah (Islamic Law), marriages between men and women standing in a certain relationship to one another are prohibited. These prohibited degrees are either of a permanent nature or a temporary. The permanently prohibited degrees of marriage are laid down in the Qur'an: And marry not those women whom your fathers married, except what has already happened (of that nature) in the past. Lo! it was ever lewdness and abomination, and an evil way. Forbidden unto you are your mothers and your daughters, and your sisters and your father's sisters and your mother's sisters, and your brother's daughters and your sister's daughters, and your foster-mothers and your foster-sisters, and your mothers-in-law and your step-daughters who are under your mother-in-law and your step-daughters who are under your protection (born) of your women unto whom you have gone into -- but if you have not gone into them, then it is no sin for you (to marry their daughters) -- and the wives of your sons from your own loins, and that you should have two sisters together, except what has already happened (of that nature) in the past. Allah is ever-Forgiving, Merciful. Qur’an.Surah Nisa 4:22 - 24)
From the above Ayah, it is clear that a Muslim must never marry the following:
A great wisdom lies behind these prohibitions on the grounds of consanguinity, affinity, and fosterage. No social cohesion can exist if people do not keep these prohibitions in their minds while contracting marriages.
Temporary prohibitions are those which arise only on account of certain special circumstances in which the parties are placed. If the circumstances change, the prohibition also disappears. They are as follows:
The word “zawj” is used in the Qur’an to mean a pair or a mate. In general it usage refers to marriage. The general purpose of marriage is that the sexes can provide company to one another, love to one another, procreate children and live in peace and tranquility to the commandments of Allah (Glory be to Him).
Through Marriage, the conjugal relationship between a man and a woman becomes lawful. It provides a legitimate outlet for recreation as well as procreation. Islam regards sex as natural and good, but restricts it to the partners of marriage so as to ensure the responsibility for its consequences.
Marriage provides spiritual, physical, emotional and psychological companionship. This companionship generates and sustains love, kindness, compassion, mutual confidence, solace and succor (sakinah). It lays a spiritual and legal foundation for raising a family. The children born of the matrimonial union become legitimate and mutual rights of inheritance are established.