The Pen Tool is a great device to know how to use in Adobe Illustrator. I have used it in two of my classes already. You can basically do whatever your heart desires with this tool. You can trace over pictures you have taken to give it a cartoonish look, or you can freely draw what you want. You can then take what you have drawn and color it in using colors of your choice.
At first the Pen Tool can be difficult to get the hang of. You can use it to create straight edged lines or curved lines. In both of my classes so far we focused more on the curved one, which you have to click and drag in a particular way that makes and shapes the line. You can also connect the lines together to make an entire closed shape, which makes it easier to color later on.
The two projects I have done involving the Pen Tool were tracing over a self portrait for Computer Graphics I and tracing over a coloring book page for Digital Illustration. In the self portrait the idea was to find the lines and shapes yourself such as the outline of the face etc. But we were also looking for areas of light and dark to be able to color in later. For the coloring book page the lines were already there for us to trace, but the coloring was more up to us to choose.