A long time ago in a galaxy far far away someone realized it would be good to have computer programs write web pages. They came up with Common Gateway Interface which is a standard way to achieve this.

Basically CGI enables a web server to run a program to respond to a request for a web address (URL). This request will probably contain information to be processed by the program (for example some parameters). The program runs independently of the web server and returns a web page (or similar output) for the web server to return to the requestor.

CGI is simple and operates well with most web servers, but can be slow because a process is started and run once for every request. An alternative called FastCGI provides a faster option with a little configuration and background reading.