Who is considered a Mahram and what are the rules of Hijab before non-Mahrams?

A Mahram is a male relative with whom marriage is permanently forbidden due to blood ties (father, brother, son, paternal/maternal uncle), foster-relations (milk brothers), or in-law ties (father-in-law, son-in-law). Marriage with them is haram. In front of Mahrams, a woman must cover her primary Satr but does not need Hijab for her face. In front of non-Mahrams (strangers), she must observe complete Purdah (Hijab). It is obligatory to cover the entire body, including the face if there is fear of fitna. It is also forbidden to sit in seclusion (Khalwat) or engage in unnecessary talk with non-Mahrams.