In child custody situations, "joint custody" usually refers to one of two possible scenarios: joint legal and physical custody, or joint legal custody.
In true "joint custody" arrangements, parents share equal "legal custody" and "physical custody" rights. This means that parents participate equally in making decisions about the child's upbringing and welfare, and split time evenly in having day-to-day care and responsibility for the child -- including the parent's right to have the child live with them. True joint custody arrangements are rare, because of their potential to cause both personal difficulties (stress, disruption of child's routine) and practical problems (scheduling, costs of maintaining two permanent living spaces for the child).
Example: Mother and Father are divorced, and agree to a true joint custody arrangement over Child. Mother and Father will work together to reach agreement on all major issues concerning Child's welfare and upbringing (legal custody), and agree to a schedule where Child lives with each parent for one month at a time (physical custody).
Much more common than true joint custody arrangements (where both physical and legal custody are shared) is "joint legal custody," in which both parents share the right to make long-term decisions about the raising of a child and key aspects of the child's welfare, with physical custody awarded to one parent.
Example: Mother and Father are divorced, and decide to share joint legal custody of Child, but also agree that Mother should have primary physical custody of Child. Mother and Father will work together to reach agreement on all major issues concerning Child's welfare and upbringing (legal custody), but Child will live primarily with Mother.