This article covers the difference between a void and a refund.
You can void a transaction if the transaction is part of a batch that has not been sent to the bank to be settled. Voids cancel a transaction before money is withdrawn from an account.
If the transaction is part of a batch that has been settled, then the money has already been withdrawn from an account. A refund reverses the transaction so that the money is put back into the account.
Voids happen instantly. Refunds can take up to 15 business days.