"Like a diamond she is carefully protected, her beauty only to be witnessed by the beholder and not exposed to the world."
The Hijab is not only the name for the material that is used to cover the women's hair (that is the khimar) but it is the condition that all Muslims (note: not just the women) should obtain when around the people of the opposite sex (this doesn't include the spouse, brothers/sisters, the father/mother, immediate uncles/aunts and immediate nephews/nieces).
This is why:
"Say to the believing men that they should lower their gaze and to protect their private organs; that is much purer for them; indeed Allah is Aware of their deeds" (Surah Al Noor : 30)
"And say to the faithful women to lower their gazes, and to guard their private parts, and not to display their beauty except what is apparent of it, and to extend their head coverings (khimars) to cover their bosoms (jaybs), and not to display their beauty except to their husbands, or their fathers, or their husband's fathers, or their sons, or their husband's sons, or their brothers, or their brothers' sons, or their sisters' sons, or their womenfolk, or what their right hands rule (slaves), or the followers from the men who do not feel sexual desire, or the small children to whom the nakedness of women is not apparent, and not to strike their feet (on the ground) so as to make known what they hide of their adornments. And turn in repentance to Allah together, O you the faithful, in order that you are successful" (Surah Al Noor : 31)
Here is some hard evidence to why the Hijab is fard (obligatory). Allah (swt) has made it very clear in the Qur'an, therefore there's nothing more any person can add or take away from it, as the guidance is there.
As Muslims we submit to Allah, so without a doubt we obey our Lord's command. He is All Knowing and All Wise.
As you can see, the first Qur'anic verse (30) indicates that the Hijab was made obligatory for men before it was for the women which is the verse 31 of Surah Al Noor.
Many people misunderstand the Hijab and think that it segregates women from society, it is 'a mobile prison' or a sign of the enslavement of women, however this is incorrect. The Hijab does not restrict a woman's position in society, but it restricts a man from viewing her as he wishes; as a sexual object. Take for example, the most successful women that ever lived.. (I do not mean Florence Nightingale, Margaret Thatcher or Elizabeth I - no offense) I'm talking about Khadija (ra) a great business woman, Aisha (ra) one of the best scholars of Islam, Umm Ash Shif'a the expert of science and medicine at her time and many others, may Allah be pleased with them all. These were women that were very educated and they played an enormous part in society
As for those who believe that the Hijab enslaves women. Is the woman who doesn't cover free? What is the meaning of being free? Is the model who shows as much of her flesh as possible for advertisement purposes 'free'? Is she not oppressed by the men that dictate to her how she must dress or undress?
Muslim women are constantly accused of being victims of oppression, but has anyone that holds such views, ever stopped to ask the Muslim woman if she feels oppressed?
The topic of freedom - if freedom always comes with restrictions, then I ask, how is that freedom? Does it even exist? If these secular societies promote freedom (and other rubbish), why are Muslim women bullied to take off that which liberates them?? Hmmm....
However, Muslim women or men do not wish to be free, we are slaves of Allah, we do as He Has Commanded, we believe life on this Earth is to worship Him alone, hoping for the greatest reward of Jannah (Paradise) insha'Allah.