Hijab is the Quranic requirement that Muslims, both male and female, dress and behave modestly. The most important Quranic verse relating to hijab is sura 24:31, which says,
Islamic scholars agree that a woman should act and dress in a way that does not draw sexual attention to her when she is in the presence of someone of the opposite sex. Some islamic scholars[who?] specify which areas of the body must be covered; most of these require that everything besides the face and hands be covered, and some require all but the eyes to be covered, using garments such as chadors or burqas.[citation needed] Most mainstream scholars say that men, in contrast, should cover themselves from the navel to the knees. see more
source: http://en.wikipedia.org/wiki/Women_and_Islam#Dress_code