Islamic rules of satr or covering for women are intended to safeguard and preserve the honor and dignity of women. Rules of covering vary according to whether the degree of risk of temptation is greater or lesser; where such risks of temptation are greater, rules of covering are stricter, and where the risk is minimal, rules are minimal.
Rules of covering are, therefore, stricter in the presence of males who are strangers (who are not related to the woman by blood, marriage or milk-relationship). For this category of men, scholars agree that the woman must cover her entire body except her face and hands.
As for males who are considered maharim (those who are related to the woman through blood, marriage or milk-relationship, and whom she can never marry) all scholars agree that she does not need to observe the above strict rules of covering; rather all of them agree that she is allowed to uncover her hair, face, hands, neck, feet in front of such relations.
Coming to the question of what she must cover in the presence of Muslim women, the majority among scholars agree that she may uncover her hair, face, hands, neck, shoulders, legs from below her knees as well as feet in front of Muslim women.