In order to accept credit card payments in your online business you need both a payment gateway and a credit card merchant account. Here’s how it works:

The customer places an order within the web site which is recorded in the secure shopping cart. At checkout the shopping cart passes the information to a payment gateway. The gateway checks with the bank that issued the customer’s credit card for authorization.
If authorized, the gateway passes the information back to the web site which tells the customer the order is approved, then passes the order information to your merchant bank.
To view or edit orders, you work through the gateway, not your merchant account. The gateway should also have what’s known as a ‘Virtual Terminal’ where you can manually enter orders.
The top three in the middle of the above diagram can be the hard part. Shopping carts only work with certain gateways which only work with certain merchant accounts. Once, my merchant bank just up and changed to another gateway – I was stuck with either changing to a new shopping cart or paying a programmer to reconfigure it.
I’ve found it’s easier to work backwards. That is, set up your payment gateway and merchant credit card account, then integrate it with a compatible shopping cart.
Your Own Merchant Account Articles
Let me put it bluntly - you can’t expect to do business online without being able to process credit card payments. I feel it’s best to have your own merchant account to accept credit cards.
Processing online credit card transactions is the same as face-to-face
credit card transactions, except that the online transactions have to
be routed through the payment networks.
Let me tell you my sad tale and show you why you need to get your merchant account online rather than at your local bank.
You need a Payment Gateway to pass credit card information between your shopping cart and your merchant credit card account.
For a larger-volume, reputable Ecommerce business you need a merchant account and gateway from your website or shopping cart.