Here are three great videos by Shane Benedict from WhitewaterInstruction.com about different types of whitewater kayaking boofs.
If you're new to whitewater kayaking, boofing is a way to get your kayak over some obstacle, usually a ledge of some sort, in a way that keeps the bow up so that you clear what is usually a reversal at the bottom and land in control, centered over your boat and without sinking the bow. You'll notice he breaks it down into three parts: the approach, the paddle stroke, and body position.
The Straight Boof:
The Into the Eddy Boof:
Straight and Fader Boofs:
I am by no means an expert on boofing, by the way - as I've said before, most of my boofs require scuba gear. :) But I think these videos have given me a much better idea of what to work on.
2014 Update: Here's a link to a newer post with a fantastic kayak boofing and flaring video tutorial.