Physical custody is simply whose care the children are under at the time. If you have primary physical custody of your children then you have your children most of the time and the other parent has them on a set schedule, generally every other weekend. Whereas if you have joint physical custody you are sharing time on an equal basis sometimes switching off every week or even every few days.
Legal custody is the decision-making power for the children. Like physical custody, if you have primary legal custody, then you are making most of the decisions for your children. While if you share joint legal custody then you and the other parent make decisions for the children together. In either event, custody schedules should be set with the best interest of the children in mind.