I'm putting together a tutorial for drawing and painting on the iPad, and I spent a few minutes tonight saving off the progress of a quick sketch, focusing on the use of layers in the Art Rage app on the iPad. "Layers" is a fairly common feature in the better drawing and painting apps on any platform/OS--from Photoshop, Painter, Sketchbook Pro, to Art Rage, and a batch of others.
Think of a layer as a sheet of tracing paper you can put down over your drawing to use the underlying line work to guide you--but with a lot more power, including blending modes, which I won't get into.
From left to right: a quick sketch to get down the figure in motion and not much else. Adding a second layer, I reduce the transparency of the first and draw--or redraw--the figure with more definition, bringing out folds in the cloth and giving the face some guidelines. Creating a third layer, reducing the transparency on the second, I now use the work in first two layers to redraw the figure with even greater detail.
Click the image for the full view: