An API (Application Programming Interface) is used in many different contexts in this industry. To clear this up with our service read below:

We offer a Common Gateway Interface (CGI). This means your site will need to contain an HTML form or link with cgi parameters appended which passes the correct parameters (and your customer) to our site for checkout. Passing in the Proper Parameters will result in your customer going through the rest of our checkout process (such as entering their credit card information), and after verification, a successful sale.

All parameters are discussed in detail within the Secure Admin Panel for Vendors.