It is cultural, not religious. Though the covering of women's hair is supported by most interpretations of the Koran/Hadith, covering the face is not. It arises from a very imprecise passage with reference to the women of the prophet Mohammed's family being 'screened'.
Islam does not directly mandate the covering of women's faces. The cultural nature of this practice is shown by the Tuareg, whose women are never veiled, but whose men often are. The Tuareg are Moslem.
