Introduction to Divs
A div is an HTML element. That's the technical definition. For the average person, a div is a box that you can use to create advanced page layouts (in theory).
The Box
The div creates a box that you can apply inline CSS to. To show the box, here's a div with a border :
[[div style="border:1px solid #000;width:100px;height:100px;"]]
box
[[/div]]
This code will create :
box