If your credit card has a zero balance when the return is processed, a credit is generally added to the account that can be applied to future purchases. If your card remains inactive with a negative balance for several months, the credit card issuer will likely send you a check for the credited funds. You can also call your credit card company to request that a check for the account credit be mailed to you, thus eliminating the wait.
If there is a balance on your credit card when a refund for a purchase from a previous billing cycle is issued, then the credit will simply be applied to your account, reducing your existing credit card balance.
If you made a purchase then carried a balance into another statement period you may be charged interest. When you make a return, that interest will not be refunded.