A merchant processor helps facilitate the secure transfer of cash between the customer and the business, while the payment gateway does the job of accepting correct or legitimate payment details and communicating approved or declined transactions, as the case may be. So, when you enter your card details on a merchant’s website, you are dealing with the payment gateway. It is when you have sent your payment that the merchant processor gets to work to complete the transaction after successful authorization from all intermediaries, which usually include banks and card networks.