Think of layers as a stack of window glass or "clear" paper. Each piece in the stack can instantly be made available for painting on by clicking it's corresponding layer representation in the layers box. This way you can paint and erase all you like on one layer without disturbing whatever you have painted on the other layers. You can change the order of the layers by holding the mouse button down on the layer you want to move and dragging it to a lower level. Also you can click on a layer in the layer box and use the little slider tool (looks kind of like a crude volume control thing) under the layer box to make the selected layer more "see through" so that what ever layers are under it show through the painted part in varying degrees depending on how transparent you made that layer. You can also click on the little "v" on a layer in the layer box to make that layer not visible so you can see what the other layers look like without that layer...if you like you can then delete that layer by clicking on it's icon in the layers box and then clicking on the delete button in the layers box. I should also mention that this is all pertaining to the Lascaux Sketch paint program, some of it it may be relevent to other paint programs, I dunno.