dotcomma.org: Introduction to CGI Concepts

You may already know what CGI is and just want to know how
to use it. But for those who don’t know, this question is a great
place to start. To sum it up, CGI stands for Common Gateway
Interface, and it is a method for creating dynamic web pages.

If that means nothing to you, don’t worry, it will.”

“When you go to a web page that doesn’t have an CGI (or
something similar) then you are given a static html page – that is,
it’s the same every time someone views it. If you reload the page
or tell a friend the address to the page it will be exactly the
same. The only way to change it is for the author of the page (or
someone else who has access to it) manually edits the page using
HTML or a webpage editor.”

“CGI changes all that. Webpages can be created dynamically, that
is, the webpage can change without the author physically changing
it. What is really happening is that the page is being created on
the server. The CGI script (or program) is generating a list of
HTML commands, which make a webpage….”