Completed: The transaction successfully processed.
Declined: The transaction did not go through successfully. Possibly the credit card information entered was incorrect or invalid, or possibly the card has a hold on it, etc. Tip: If you click on the Transaction details you can scroll down and sometimes see further details of what might have caused the Decline.
Refunded: This means you issued a refund. If you manually change the status to Refunded, you would still need to officially issue the refund.
Voided: This means that you Voided (or blocked) the Transaction from actually posting. No need to refund, because the funds never with withdrawn from their account. Again, simply changing the status does not officially Void a Transaction, you would need to actually click the Void button.