There are a number of different ones that you can use.
3rd party accounts
and
1st party
===========
3rd party = Paypal, 2CO, Egold, etc
1st party = BluePay.com , Authorize.net, Linkpoint.com (3 major ones right there)
and then there are some other ones.
Each Merchant account you have needs to have its own gateway. Most of the time you get charged for both of them. A "gateway" fee, and then a "statement" fee.
You will have to pay a transaction fee for every time you run a card plus a percentage of the total transaction.
In order to get one you will need decent credit or have proof of having one previously.
Was there anything specific you needed to know?