"Legal custody" gives a parent the right to make long-term decisions about the raising of a child, and key aspects of the child's welfare -- including the child's education, medical care, dental care, and religious instruction. In most child custody cases, legal custody is awarded to both parents (called "joint legal custody"), unless it is shown that one parent is somehow unfit, or is incapable of making decisions about the child's upbringing. Legal custody is different from "physical custody," which involves issues such as where the child will live.
Example #1: Mother and Father have divorced, and share legal custody of Child. This means that Mother and Father share equally in making important decisions concerning Child's upbringing and welfare.
Example #2: Mother and Father have divorced, due in large part to Father's substance abuse and addiction. Mother seeks and is granted legal custody of Child. This means that Mother alone has legal authority to decide key issues related to Child's upbringing, and Father has no legal right to participate in the decision-making process.