There is the default layer which you can call the backlayer and the forelayer (only one).
The forelayer allows you to easily draw moving objects without redrawing the background.
You can't switch the layers.
Here is an example that uses the forelayer:
http://www.basic4ppc.com/files/BouncingSmiley.zip
The forelayer was introduced in version 3.00 and libraries where not supported at that time.
It is now possible to use ImageLib (and other image libraries) to do such things.
However the forelayer is still useful as it is very simple to use.