The answer to the question is very simple - Muslim women observe HIJAB (covering the head and the body) because Allah has told them to do so.
"O Prophet, tell your wives and daughters and the believing women to draw their outer garments around them (when they go out or are among men). That is better in order that they may be known (to be Muslims) and not annoyed..." [Noble Quran 33:59]
Other secondary reasons include the requirement for modesty in both men and women. Both will then be evaluated for intelligence and skills instead of looks and sexuality. An Iranian school girl is quoted as saying, "We want to stop men from treating us like sex objects, as they have always done.
The word "Hijab" comes from the Arabic word "hajaba" meaning to hide from view or conceal. In the present time, the context of Hijab is the modest covering of a Muslim woman.
Islam has no fixed standard as to the style of dress or type of clothing that Muslims must wear. However, some requirements must be met. The first of these requirements is the parts of the body which must be covered. Islam has two sources for guidance and rulings: first, the Quran, the revealed word of Allah and secondly, the Hadith or the traditions of the Prophet Muhammad (peace and blessings be upon him) who was chosen by Allah to be the role model for mankind. The following is a Tradition of the Prophet:
"Ayshah reported that Asma' the daughter of Abu Bakr came to the Messenger of Allah (peace and blessings be upon him) while wearing thin clothing. He approached her and said: 'O Asma'! When a girl reaches the menstrual age, it is not properthat anything should remain exposed except this and this. He pointed to theface and hands." [Abu Dawud]
The second requirement is looseness. The clothing must be loose enough so as not to describe the shape of the woman's body. One desirable way to hide the shape of the body is to wear a cloak over other clothes.
Thickness is the third requirement. The clothing must be thick enough so as not to show the color of the skin it covers or the shape of the body. The Prophet Muhammad (peace and blessings be upon him) stated that in later generations of his Ummah there would be
"women who would be dressed but naked and on top of their heads (what looks like) camel humps. Curse them for they are truly cursed." [Muslim]
Another requirement is an over-all dignified appearance. The clothing should not attract men's attention to the woman. It should not be shiny and flashy so that everyone notices the dress and the woman. In addition there are other requirements:
Women must not dress so as to appear as men.
"Ibn Abbas narrated: 'The Prophet (peace and blessings be upon him) cursed the men who appear like women and the women who appear like men.'" [Bukhari]
Women should not dress in a way similar to the unbelievers.
The clothing should be modest, not excessively fancy and also not excessively ragged to gain others admiration or sympathy.
For both men and women, clothing requirements are not meant to be a restriction but rather a way in which society will function in a proper, Islamic manner.
