Logic behind directories based web apps

I have been developing web apps in php which contain multiple users, or elements, and what I do is to use one file and identify the element by giving a php get, for example: http://ift.tt/2qTwosN But I have seen many web apps that instead of doing that, they do it through directories, for example: website.com/user/1 So I was wondering what was the logic or how is it made an app based on directories instead of gets and posts. Thanks