Well, back in 2003 I made a sticky thread about it in the:
Lounges > Maya Basics and Newbee Forum
Its the 3rd thread down from the top titled:
EXPLANATION: Render Layers Pass Control
Its a bit out of date, so for you I would suggest checking out this more recent (Maya 7) flash video overview direct from Alias:
Alan is our resident Rendering Moderator but as a TD at Framestore he is currently busy working on Narnia 2: Prince Caspian.
I recently got my certifications updated for Maya 8.5, so maybe this weekend I can update that thread with a newer mini tutorial covering the updated features of the Render Layers and Material Overrides abilities.
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS