Praise be to Allaah.
We have already spoken about the fact that it is obligatory for women to cover their faces in Question #2198, so please refer to that question.
With regard to the niqaab, this is a cover or veil with a hole in it for the eyes. It is a way of dressing that was known to Muslim women at the beginning of Islam, and it is mentioned in the hadeeth of the Prophet (peace and blessings of Allaah be upon him) in which he mentions what women in ihraam (for Hajj or Umrah) are not allowed to do. He said: The woman in ihraam should not cover her face with a veil or wear gloves. (Reported by al-Bukhaari, 1707). Some of the scholars mentioned that a woman may uncover her left eye in order to see where she is going, and if necessary she may uncover both eyes. The opening should only be wide enough for the eyes, and the woman should beware of wearing a niqaab with wide openings lest the beauty of her eyes becomes apparent and the niqaab becomes a means of temptation instead of a veil covering her beauty and adornment.