What is the difference between the niqab, the khimar, and the hijab, and what is the legal ruling on the obligation of each, specifying its generality and specificity?

The three terms: hijab, khimar, and niqab, all denote covering and concealment. Hijab is the most general of them, encompassing the covering of all that a woman must conceal of her body and adornment. As for khimar, it is what a woman places on her head and lets down over her face and neck, as indicated by the Almighty's saying: "and to draw their khimars over their bosoms." Khimar is more specific than hijab, and likewise niqab, which is specifically designed to cover a woman's face with two openings for the eyes. It is obligatory for a woman, if she wears the khimar or niqab, to cover the rest of her body in front of non-mahram men.