A refund receipt reflects a refund that you gave to a customer. In many cases, refund receipts will be used to document credit card refunds, but they can also account for other types of refunds to customers.
When a refund receipt is created, QuickBooks will debit (lower) the revenue tied to the items you are refunding. The system will also credit (also lower in this case) the bank account or undeposited funds account that is used for batching payments.
A credit memo reflects a "credit" on a customer's account. Often, credit memos are the result of an unpaid invoice being "written off." However, they can also be used to reflect that you have decided to credit a customer for a particular purchase. For instance, a member could not make a class, and you decided to leave a credit on their account towards a future class. In the case of a paid invoice or sales receipt, this reflects the fact that you are holding on to the customer's money in lieu of refunding it.
When a credit memo is created, like a refund receipt, QuickBooks will debit (lower) the revenue tied to the items you are refunding. Unlike a refund receipt, the system will credit (lower in this case) your accounts receivable, not your undeposited funds account.