Photoshop layers

We've started looking at making picture adjustments using layers.
It's an important technique in Photoshop, and prevents you from destroying your valuable photographs, so that can only be a good thing.
Here are a couple of videos that might help:

http://www.youtube.com/watch?v=4ol5_M25ZKM

http://www.youtube.com/watch?v=iLchFwmLu_g

http://blog.infiniteskills.com/2012/08/adobe-photoshop-cs6-tutorial-layers-101-an-introduction/

Editing with layers and levels in practice:
http://www.screencast.com/t/9rH9QBPCpy