The use of CGI is waning. Basically, CGI is referring to programs written in a language called PERL, which is quite robust and made for the web.

Until PHP came along, PERL was the best (arguably) way to write a website if you had any dynamic content, a database, or needed programming type of control.

With a PERL script you can do just about anything you can with any other language, and often times you don't have the hugely dynamic URLs that other languages will produce. That is not to say you won't get that with CGI programming as well though.