According to the California Department of Child Support Services, child support payments cover a parent's contributions to a child's medical and living expenses. Child support is determined by a court ruling. The court can rule that either or both parents are required to provide child support payments. In general, both parents are under a legal obligation to financially contribute to their child's support.
DadsDivorce.com notes that child support is intended to cover expenses such as medical insurance, shelter, clothing and food. Some expenses, such as education, uninsured medical expenses, and expenses related to extracurricular activities, are not included in the child support payments unless specified by a settlement agreement or court ruling. In addition to child support amounts, which are an estimate of what the child needs to cover basic living expenses, parents may be required to spend additional funds during their time with the child.
Wikipedia notes that child support can be paid to either parent. A common arrangement is for the non-custodial parent to pay child support to the custodial parent. In the event that the parents have joint custody of a child, the parent with a higher income may be required to pay child support to the parent with a lower income.