There are two different types of "joint custody." You can have joint legal custody and/or joint physical custody.
Joint legal custody means that both parents have an equal voice in making decisions in respect to their child, though one parent may still have primary physical custody. So, both parents have equal ability to make decisions, but the child lives with one parent (and usually has visitation with the other). You do not need the other parents permission, but you need to be able to make the decision together.
Joint physical custody means that both parents have equal rights to the physical custody - sometimes people will confuse joint physical custody with divided custody, meaning that the child is with each parent for approximately an equal period of time, i.e., one week with mom, one week with dad. But joint physical custody would mean that the child could be at either parent's home at any particular time as the child can go back and forth.
Both types of joint custody requires that the parents be able to work together, especially in making decisions otherwise it will not work and most likely the matter will end up back in court and in most cases the court will then dissolve the joint custody and award sole custody to one of the parents.