A Div tag can be used for formatting a block of text, even if these texts are included within some other tag. One or more Paragraphs can be aligned with the same alignment using div tag. Many other things can be done using div tag.
An "AP Div" is just what Dreamweaver calls a div with "absolute positioning", which means you can specify exactly where you want it to appear on the page. A standard div will just appear in the normal flow of the page, but is useful because you can apply whatever styling you want to it.
This AP Div tag can have anything placed into it and then you can position it where ever you want it. See following Examples of Image.