A niqab is a cloth that covers the face of a devout Muslim woman. Specifically, a niqab is worn below the eyes, covering the nose and mouth.
While some observant Muslim women wear a burka, which covers the entire body, and others wear only a hijab that covers their hair and sometimes their neck, still others wear a niqab. The niqab leaves the eyes uncovered, and it's almost always worn with a separate scarf covering the hair. Women who follow this tradition wear a niqab when they are in public or in the company of men they aren't related to.