Payment gateway enables internet merchants to accept online payments via credit card and e-check.
http://www.authorize.net/

Authorize.Net is a payment gateway service provider allowing merchants to accept credit card and electronic checks payments through their Web site and over an IP (Internet Protocol ...
http://en.wikipedia.org/wiki/Authorize.net

Developers can easily integrate to Authorize.Net with our freely available sample code and documentation.
http://developer.authorize.net/

Authorize.Net implementation guides and documentation for the Simple Integration Method (SIM) and the Advanced Integration Method (AIM)
http://developer.authorize.net/guides/

Merchant Focus is proud to offer access to the highest quality credit card processing and payment solutions.
http://www.merchantfocus.com/