Common Gateway Interface (CGI) Explained

We often hear or read the term CGI while browsing across the web. What is it really?
Keeping first things first, we should know that "CGI" is a well accepted standard.
It is used for interfacing a server and external applications.
As we all know, originally web servers were used to serve static HTML pages & static files. But, web servers are usually installed on powerful computers. So, it makes sense to use their full potential by making them serve dynamic & more interesting web pages.

This is the reason why CGI Specifications were created.

[Thread to be continued..]