I wanted to keep track of this article for future reference. We found it helpful when doing a short unit on caricatures. It is from Tom Richmond, who is an artist / illustrator who works for MAD Magazine.
His article about caricatures and the 5 basic shapes is very useful for newcomers to the style of cartooning and dramatizing features for caricatures. If you want to learn more, I suggest following his blog, which is updated frequently.