The div element stands for "Division", all it does is split up your document (or existing divisions) into divisions.
Code:
<div>This is one division.</div>
<div>
Here is another division.
<div>Here is a division inside another division.</div>
</div>
It's mainly used together with JavaScript and CSS where you can target groups of content or code through them.
You'll also stumble upon another element called "span" (unless you have already), these are used for the same things as div's (pretty much) but they are in-line. I'll explain those things (if you want me to) at another time though, I'm at a hurry and have to go. :(