Ever wanted to make your drawings look more realistic and three-dimensional? Ever watned a circle to look more like a sphere? Well, all you need to know is the basics of shading and the different types to use for different effects. And then practic, practice, practice.
I'll describe the different types of shading used by artists, though normally a cartoon won't have much more shading than a few highlights or outlining shadows because it's kind of hard to animate shadows.
Rub Shading is probably the most commonly used, and most messy, form of shading. I call it rub shading because you rub the side of the pencil or charcoal chalk on the paper to give it an almost "crayon"-looking effect. The harder you scrape, the darker the shade and the more solid too. God shading to use for a quick sketch, or to fill in space quickly without having to sharpen your pencil so often.
Linear Shading is commonly used by comic book artists. This type of shading uses distance and thickness of lines to give a blending effect in any direction. Start with a very thick line, almost like a big dark rectangle. Then leave a sliver of space and put another, less-thick, line down. Continue this pattern, leaving more space in between lines as you shade closer to the lightsource, and make sure the lines get thinner until you fad away into white.
Cross-Hatching can be very useful to set an angry mood to a picture or to blend an object into oblivious black. It's basically just linear shading, only you're crossing one linear shade set with another in a "X" formation.
Volumetric Shading beautifully blends colors, especially, and is probably more meaningful for them. Using the tip of a pencil or pen, run back and forth on the area of the paper making a dark shade, then proceed to move the pencil in this almost "zig-zagged" fashion toward the light source letting up on the weight of the pencil as you get closer to the light. I like using this shading the most because it blends so well.