Joint custody is a broad term which in legal terms means that both parents must agree on major decisions which affect their child. Neither parent is allowed to decide on child related matters unilaterally without consulting the other parent. If there is a disagreement they must work together to solve the problem otherwise they may find themselves back in court.
Sometimes parents are unable to agree on anything, and this may result in a court making an order for sole custody. Joint custody only works when both parents are willing to work together, at least where things really matter for their child. Sometimes parents who cannot agree on anything instead decide they will each make decisions on certain subjects and this allows them to maintain a joint custody regime without having to work together as much.
Joint custody can be vague in some situations. For example, many parents assume that if you have joint custody then you are sharing your children on a 50/50 basis. This is not necessarily the case. Joint custody simply refers to decision making powers with regards to major decisions for your child. It is in fact common for parents to have joint custody where one of the parents has “primary residency” and the other parent has access on an occasional basis.