Web FAQ 5.01. What are CGI scripts?
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CGI scripts are executable programs that are referred to on your Web page but run on the server. The output of these programs can also be directed to a Web page. For example, let's say that you wanted to make a guestbook on your Web page, where a form is used for people to fill in data. This data is to be then displayed on a guestbook page. Basically, you will need to create a CGI program that the server executes when somebody fills the form and hits the submit button. This program receives the data, and then displays them on the guestbook page as needed.
The Penn State directory is an example of a CGI script. A user fills out a Web form and submits the data to a CGI program which queries the directory database and presents the output in HTML format.