Layers are very very very helpful for those of you who want your colors to come out clean and in the lines. So, without futher ado, here is a tutorial I found that can show you how to use layers.

http://iit.bloomu.edu/vthc/Photoshop/Basics/layers.htm

***You can also set the layer to multiply (you can find the scroll down menu at the top of the layer box) which will allow you to color over your line art and the color will go under the lines. So if you color outside the lines you can first of all see that you did, and second you can go through and erase it with the eraser tool or if you colored outside a large area you can use the lasso tool (I perfer the lasso tool on gimp over photoshop, so when using photoshop it's easier for me just to use the eraser tool), select the section by putting a dotted line around it with the tool and then hit delete.***