CGI is an acronym for Common Gateway Interface. Even if it often is mentioned as a program language it is accually a protocol in which you may use several program languages. The ones most frequently used are Perl, C and shell-script. CGI have been around for several years and is therefore very stable and well tested. CGI is a server-side scripting language and is normaly put on a seperate area on the server. This area is normaly named CGI-bin. There are few ISP's giving you the option to use CGI. This is because it's easy to use a 'abuse' a script and harm the server.

CGI is often used in seach engines and database-systems and is used on sites to extend the scriping language mainly used on that site.