Doo implements the model-view-controller (MVC) design pattern which is widely adopted in Web development. MVC aims to separate business logic from user interface considerations so that developers can more easily change each part without affecting the other.

Doesn't make sense to you? Nevermind, just imagine MVC as 3 folders, and each folder is for you to store files for 3 different purpose:

So when a user access your Doo application, the files in the 3 folders will be used. The flow of request handling is like the diagram below:

For more infomation on MVC, you can read it up at Wikipedia. http://en.wikipedia.org/wiki/Model-view-controller