Messages are simply a 32bit number designating some event. They get queued up and a window eventually takes them out of the queue and reacts to them. Let's move on to Handles. Windows is very much object oriented. He uses handles.
Handles are a way to reference different windows objects.
You can have handles to windows, handles to files, handles to allocated memory, handles to images, etc. You can think of them as pointers. You must create them some how. And when you are done with them, you must destroy them. If you don't you will end up with what is called a resource leak.