CGI is the glue between a Web server and server-side interactive programming. The input to a CGI program, if any, originates with forms, imagemaps, or URLs, and as such can be from just about any source. The output from a CGI program can be any of the content that a Web browser can display, from dense text to custom-generated graphics.

A CGI program cannot update a Web page in place on a browser because the HTTP connection does not have a method for replacing part of an object without sending an entire new object.
Even if HTTP could update a Web page in place on a browser, HTML doesn't support on-the-fly page updates. This is a limitation of the protocols and languages involved.
Aside from that limitation, a CGI program can generate virtually any object that can be displayed on a Web browser. In fact, it can even generate things that cannot be displayed on a browser. The possibilities are limited only by your imagination.
Here are some examples of CGI is being used in working projects.

Click the Exercise link below to examine a Web site that uses a CGI program.
Perl CGI - Exercise
In the next lesson, you will find an introduction to Web servers that includes necessary information about a server's architecture, so that you can write CGI programs that interact with the server.