Photoshop uses layers so you can edit induvidual elements of a picture and effecting the other parts of the picture.

Press F7 and you should see this window



This window shows you all the layers in the current project and in what order they are in, top most being at the front, lowest at the back. You can click and drag these about to reorder them. It also shows you what effects have been added to that layer.

In that picture, Layer 5 is highlighted and has a paint brush in the box on the left showing that layer is selected for editing.

If you click the little eye you can hide/unhide layers.

Now I want to erase the toon ecossie from the picture below.



But as layer 5 is selected (in this case the background) it will only erase that and not the cossie

So I have to select the layer 3 (the cossie) and try again;



hey presto, can now erase the cossie.

Hope this helps.