This paragraph demonstrates how you can use DIV's. It uses a combination of DIV's, ID's, and the CSS "float" property for positioning. It's a very standard layout for pages that have only three sections (i.e., a "banner" across the top, a menu bar down the right side, and content on the left).
ID's are useful for positioning because you can only use an ID once on a page (i.e., I can only have one "menu" section per page; if I were to call another DIV "menu," it wouldn't show up). Classes can be used as many times as you want on a page, so you can use them for styles you want to apply more than once.