[ from the CGI reference http://hoohoo.ncsa.uiuc.edu/cgi/overview.html ]

The Common Gateway Interface, or CGI, is a standard for external
gateway programs to interface with information servers such as HTTP servers.
A plain HTML document that the Web daemon retrieves is static,
which means it exists in a constant state: a text file that doesn't change.
A CGI program, on the other hand, is executed in real-time, so that it
can output dynamic information.