Layers may be thought of as sheets of transparent paper with different pieces of the overall drawing on each. Together the layers form the overall drawing, but each layer may be edited independantly of the others. Layers are controlled via the Layers dialog (View -> Layers...). Each layer has three characteristics represented by the eye, pen, and printer icons respectively:
Visible: allows the in/visible.
Editable: determines whether changes may be made to the layer.
printable: allows the layer to be printed.