Hijab in the Qur'an:
“And tell the believing men to lower their gaze and be modest. That is purer for them. Lo! Allah is Aware of what they do. And tell the believing women to lower their gaze and be modest, and to display of their adornment only that which is apparent, and to draw their veils over their bosoms, and not to reveal their adornment save to their own husbands….”24:30-31.
“Such elderly women as are past the prospect of marriage, there is no blame on them if they lay aside their outer garments, provided they make not a wanton display of their beauty; but it is best for them to be modest and God is One who sees and knows all things.”24:60
“O Prophet! Tell thy wives and daughters and the believing women that they should cast their outer garments over their persons when abroad that is convenient that they should be known and as such not molested. And God is Oft-Forgiving, Most Merciful.”33:59.
What is it?
In brief, the hijab is often described as the modest dress of a Muslim woman, or more specifically, the hair-covering. Islamic teachings tell us that the Islamic modest dress for a woman should cover all but hands and face in clothing that does not conform to the body shape. Some sources prescribe the dress more strictly by limiting fashion and color and/or suggesting the covering of the face. It is not a uniform, thus one can find a lot of variation of styles of Islamic dress, as well as differing interpretations in the details. For example, many people interpret the wearing of a scarf with long-sleeve blouse and long pants that do not cling to the body as within the basic requirements of hijab. But some people or cultures feel differently and interpret it more strictly, perhaps preferring skirts or dresses to pants, or perhaps preferring mute colors to brighter colors, and so on.
Islamic modest dress applies to men as well, but in a different fashion. However, the nature of modesty and Islamic dress for men is often neglected, especially in view of the obsession that sometimes exist over the dress of women.
Hijab also has a philosophy or spirit to it, in addition to the physical nature. That is, as with almost everything in Islamic practice, there is an outward, physical nature and there is an inward, spiritual nature. The spiritual nature of hijab is the spirit of modesty - not only in how we present ourselves to the world, but in how we take in the world. The modesty of presentation includes not only dress, but demeanor and behavior. As for how we take in the world, this means how we observe the modesty of the eyes, ears, etc. - do we turn our eyes and ears away from things that violate the spirit of hijab and modesty or do we allow our eyes and ears to take in all things?
The spirit of hijab is very important, for if one acts only on the outward nature without acting on the inward nature, then one really misses the point altogether, and really misses out on the essence and heart of what hijab is all about.
The same is true for every act. Take, for example, the prayer. One can perform the mechanics of prayer and totally miss the inward aspects and thus benefit little from the prayer. The inward aspects of the prayer - the attention to God, the meaning of the words, and so on, are the true prayer.
A Muslim is one who submits to God, while a momin is someone who is a believer. Adopting a practice one knows is right even if she doesn't feel her heart is totally in it is an act of submission; it is an act of a Muslim. When one takes this first step, it is not uncommon that God returns the act by elevating one to the state of a believer so that now one does the act not out of submission but out of belief, and eventually maybe even out of love, desire, etc.
Why is it?
God knows best why He prescribes anything for us that He prescribes, but as can be seen in the ayahs mentioned above, and in the teachings of Islam preserved elsewhere, the purpose of Islamic modest dress is modesty and to be recognized as upright folk and be afforded the respect of an upright person.
Why is it that the modest dress for a woman includes covering the hair? In essence, it is due to what is her 'awra, or what is her beauty that particularly appeals to the opposite gender.
It may seem extreme, but hijab really is not extreme. In the context of past history, the modest dress of Muslim women is not unusual. Nor is it restrictive - it does not hinder her from most any action or movement she may want to do, although in reality, the Muslim (and other) societies have sometimes hindered her from some actions and movements that are not warranted by the minimum requirements of Islamic dress. It may seem different than what is common today in the Western world, but that does not make it extreme or prohibitive. Extremism and undue prohibition come from hard hearts, abandonment of spirit for letter only, and intolerance. Hijab in itself has none of these traits, but instead has at its core the ideals of equity and justice, respect, upright behavior and demeanor, and inclusiveness.
What is wearing hijab like?
Sometimes women will say they will wear the hijab later on when their spirituality matches the action - i.e., they say they are not pious enough to wear it and to wear it would be hypocritical. But this logic is erroneous. Whatever is required by God is required by God and it is not hypocrisy to follow the requirements with the intention of following the requirements, even if the spiritual self isn't in the ideal place. Even in prayer, often the heart is not in the right place, but that does not excuse one from the prayer. No one says that she should stop praying until she becomes more pious, so neither should such an argument be used about hijab. The reality is that if we sincerely adopt a practice often the heart will follow. Sometimes it is not for the heart to lead us to piety when our actions are against it, but rather it is for the heart to follow the actions on the course of seeking piety by submitting.
Hijab is one aspect of submission or belief in God. But like any other practice it is not the whole picture for any person. When we see people, Muslim or anti-Muslim, obsessing over hijab, it is a symptom of a larger problem. Sometimes we see Muslims obsessing over hijab - often this is a symptom of obsession over letter of law while neglecting spirit of law. It is also often a symptom of anti-feminine leanings that belittle the full personhood of women and objectify them just as much as those who obsess over the dress of women in the other extreme of immodesty as opposed to modesty. Sometimes we see anti-Muslim folks obsessing over hijab, too, - and this is often a symptom of intolerance us vs. them ideology, ignorance, and seeking to cause division and subjugation.
In practice, wearing hijab is like wearing any other clothes - it feels as natural as any other clothing. A person who is accustomed to wearing the hijab does not feel fully-dressed without it and in her day-to-day activities feels normal and does not feel different from someone else. At times, however, someone may react to her in a way that may remind her that she looks different than others are used to. But more of than not, she will go through her day without incident. Sometimes these reactions are positive - small acts of positive chivalry like opening a door or avoiding foul language, for example. Sometimes they are negative, such as a dirty look or bad word.
When someone first puts on hijab, however, it feels different. It can be hard for her to feel comfortable in a new style of dress. Also, she is often hyper-aware of any reactions and more self-conscious and concerned or afraid of negative reaction. On the flip-side, she is often also more aware of her dress as being potentially representative of Islam and Muslims and may be more modest in her behavior in reaction to her change in dress.
Some people avoid wearing hijab even if they believe it is right out of fear. For most women, however, this fear is out of place. God is the source of our well-being and sustenance. If we know this to be true, then we should also understand that we obtain our sustenance not through an employer, but through God. And we obtain our well-being from God also. If we wish to secure our well-being and our sustenance, then the best path is that path prescribed by God, not by following some path other than that prescribed by God. Some women certainly do face obstacles of many different types in regard to employment, relations, or safety when it comes to wearing hijab. But there are very very few women who would be turned down by every decent employer in their chosen profession - usually it is just one or two that have wrong attitudes. Similarly, a few women sometimes face danger or other hardship due to wearing hijab. But the vast majority of women will go their entire lives without ever being in danger because of it, and many having even enjoyed some measure of protection because of how people responded to her positively. If hijab closes doors, it also opens them. And ultimately, God is the All-Powerful and when we put our trust in Him and follow the path He guides us to, we cannot be disappointed.