I've just learnt it over time - the box model is fairly simple but the main reason people find it hard is because body doesn't visibly break the model.
Really, if you give body a margin and a background you should see it surrounded by a white strip. However this isn't the case - body's padding is the same as margin. This establishes a few incorrect things about the box model.
I usually think about it like this:
- margin = spacing around the box;
- border = the edge of the box;
- padding = space inside the box.
