I'm willing to bet that if you're reading this, you're a somewhat seasoned web developer but by some requirement, you are forced to create a CGI script. This may be required to add a plugin to your E-commerce platform, or a request by a really old-school, out of practice web project manager!
There are definitions all over the place for CGI, I'm not going to duplicate those efforts. Instead, I'd like to explain why the term CGI is still around.
The short answer is, because a CGI can be considered a general term for a script, written in any language, that can be interpreted by your web server and passed along to an appropriate interpreter (PHP, Python, Perl, etc.)
For instance, the following is a perfectly valid CGI file:
#!/usr/bin/php
echo 'hi, this is a CGI script'
Call it hello.cgi, and depending on your server setup, you've got a page that can be served by your web server, and accessed at http://www.blahblah.com/hello.cgi
As you've probably guessed, the very first line of the CGI file tells the web server which interpreter to use to run the file.
So, next your E-commerce platform asks for the path to a CGI file for a custom shipping plugin, you can be at ease, knowing that your language of choice will most likely fit the bill.