This is part of the Illustrator CS6 Lesson Plan.

3.6 Demonstrate knowledge of layers and masks.

Layers
Layer Basics
Editing Objects, Layers, & Groups
Understanding Layers and Stacking Order Video

Clipping Paths and Opacity Masks
Clipping Masks Basics
Mask basics
Using Clipping Paths and Opacity Masks
Clipping Masks and Compound Paths
Basic Clipping Masks Video
Cropping Images with Clipping Masks Video

Exercises
Line Art Portrait
Broken Mirror