Q - php-apache _ php-cgi

Hello, I'm kind of new to serverside management and got a little question. PHP can run on a server in different ways; as an Apache-module and as a cgi-module. - What is the biggest diffence between these two ways of running? - When I write scripts for both do I have to pay attention to something or keep an important aspect in mind..? I hope you people can clear this out for me.... Thanks in advance, Bart