CGI stands for Common Gateway Interface.
It is not a program at all, but a set of rules for the way a Web server can deliver information to a program running on the same machine,
and receive information from the program when completed.
For example, perhaps a user will fill out a form on your Web site requesting more information about your products or services.
He types his name, address, and so on into fields on an HTML form, then submits the form.
The ACTION parameter on the form identifies a program on the server.
The browser includes all the form values in the path and file name it sends to the Web server, like this:
GET /cgi/info-request.cgi?name=Tom&phone=2990633 HTTP/1.0
The Web server starts the CGI program running (request.cgi) and sends all the form values to it (name=Tom&phone=2990633).
The CGI program does something useful, such as updating a database, or sending email, then writes out HTML. The Web server sends the HTML back to the browser to be rendered for the user.
You have to write all the HTML for the whole page, even if most of the page is the same throughout your site.
If you change the look of your site, you will have to change the CGI code to generate the new look.
ACTION parameter : The attribute on an HTML FORM tag that names the program to be run on the server when the form is submitted by the user.

It is challenging to maintain state with CGI, but it can be done.
Problems regarding CGI will be discussed in the next lesson.