Common Gateway Interface (CGI) is used to generate dynamic webpages by calling external scripts to generate the actual webpage. Perl, Ruby and Python are common languages to use for CGI scripts.
CGI is enabled on Supporter accounts only. CGI scripts are executed via mod_fcgid and suEXEC. This allows for good performance while at the same time maintaining a high level of security. Scripts has to be modified to run with fcgi, or they have to be executed via a wrapper script. Normal CGI scripts will not work without some modifications.