Short Answer

The difference between a refund and voiding comes down to timing. If payment has been settled (as in, have you received the funds already), then you would issue a REFUND. If the transaction has not been settled, you would VOID the transaction.

More Information

In practice, both actions will result in a very similar outcome from the perspective of the payer. When the payment information is input online, their issuing bank (a bank or financial institution that provides lines of credit through card associations) will send the funds to