What is a layer in Blender?

And what is the best tutorial to watch to teach me about layers?

http://asitssfl01.ase.tufts.edu/blackboard/DR21/BlenderLayers.html

It’s mostly used to separate elements of the scene for rendering and composting them differently.

From the manual:
http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Post_Process/Layers

A quick tutorial on one of the many uses for it:

or take a look at any one of a number of the tutorials featuring composting.