All dependent children have a legal right to be financially supported by their parents. When parents live together with their children, they support the children together.
Parents who do not live together often have an arrangement in which a child lives most of the time with one parent. That parent is said to have custody of the child. This arrangement can be written in a separation agreement or court order (sometimes called legal custody), or may occur without a written agreement or court order (sometimes called “de facto” custody).
Either way, the parent with custody has the main responsibility for the day-to-day care of the child and has most of the ordinary expenses of raising the child. The other parent should help with those expenses by paying money to the parent with custody. This is called child support.