CGI, standing for Common Gateway Interface, is a computer language that takes information, changes it according to the script, and outputs it in the new form. CGI itself is not a language, but is written from other languages. Perl and C are the two most common languages to write CGI scripts.

All forms need a CGI script to take the information that was inputted into the fields of the forms and transform it to whatever the creator would like. For example, a site creator could create a script that takes the users name and outputs it in a letter that is personalized with the users name.