CGI, Common Gateway Interface, includes programs that run on the server to enhance the quality and functionality of a web site.
CGI extended definition: A set of rules that describe how a Web Server communicates with another piece of software on the same machine, and how the other piece of software (the ‘CGI program’) talks to the webserver. Any piece of software can be a CGI program if it handles input and output according to the CGI standard. Usually, a CGI program is a small program that takes data from a web server and does something with it, like putting the content of a form into an email message, or turning the data into a database query. CGI “scripts” are just scripts which use CGI. CGI is often confused with Perl, which is a programming language, while CGI is an interface to the server from a particular program. Perl is an application of CGI, as well as MIVA, Python, PHP3, and other scripting languages.