CGI in 200 Words or Fewer
What is a CGI program? It is a program that runs in response to a Web request. The program writes out HTML code; the Web server sends this HTML code down to the client browser. Thus, a CGI program is really just a program that dynamically builds HTML pages.
Because a CGI program runs each time a user requests the page, the CGI program can customize the page for each user. More power comes, however, when you include an HTML form. A form on a Web page can have the name of a CGI program as an action element. When the user fills in the form in the browser and clicks the Submit button, the information gets sent right to the CGI program. The CGI program can easily discover what information the user typed in and use this information to build a custom Web page.