A big part of working with advanced graphic editors is working with layers, and if you know what they are and how to use them, it can make your projects easier and more effective.
Layers are like a collection of individual images all contained within one main image.
These images can be edited and manipulated seperate of each other, or can be combined with one or more other layers to form a new consolidated layer. An easy way to visualize a layered image is to think of it as a stack of transparencies, each with something different drawn on it. If you look at the stack straight on, you'll be able to see objects on the bottom as long as there's a clear spot through the other transparencies on top of them. Well, there are times that you should and times that you shouldn't use layers. If you're making a simple clipart image or other relatively simple graphic, then there's probably no need for you to put things into different layers.