It depends on the court order. Usually it covers one-half of the needs of the child, which will change over 18 or 21 years. The court will look at the income of both parents, and try to provide payments which will give the child the lifestyle that he or she is used to. If one parent is not working, that parent may have to get a job, so childcare expense must be provided for.
If you want to elaborate, the child needs food, clothing, shelter, education, and medical care.

When the custodial parent receives child support, he or she can use it in any way that assists the child, including not only specific things like clothing and normal entertainment expenses for the child, but also general expenses, such as rent and groceries.

As long as the child support money isn't spent on things that only benefit the parent or others, the custodial parent is the one to decide what expenses should be met with the child support funds.

According to the law 1/2 of the cost of living for the child, however that percentage can be more or less depending on the earnings of each party and if there is another biological child in the home (from another reletionship). Housing, food and clothes. An addtional clause can be added to the order for "others" This usually happens when the child is older. It's a lifestyle thing. School clothes, tuba lessons ect. The custodial parent does not have to "prove" that each and every cent is for that use. In our state, michigan, the non-custodial parent pays back the custodial parent. Medical is a different part of the equation, but is added to the child support payments. The additional clause can be thrown out of court if the party can't afford it or if its deemed "un-needed" by the child. the only time I've seen it not work like that was when 1 the parent went and did it and wanted reimbursement ( although most of the time this is thrown out of court) or 2 its a medical thing, like braces (most of the time those are an elected item, not medically needed for the child to survive) Hope this helps!