Cool. So check this:
http://img193.imageshack.us/img193/2097/edgesr.jpg
Basically, along the line where the shadow side meets the light side, you have a lot of potential for edge variation. So it'll go from hard, to soft, to hard again. It's called a lost and found edge.
If you have it hard all the way down, it tends to give a flatter look. Mind you, that's completely fine for cartooning. If you're going for cel-shading then you don't have to worry about this stuff at all. But as soon as you're doing any painting at all, it's really important to take care of your edges.
Hope that's helpful anyway.