Brad King has been posting some great “getting started” introduction tutorials for ZBrush, and these two videos look at everything dealing with masking with Zbrush 4.

“In Part One of Masking with ZBrush 4 we look at some basic masking elements. We begin with a basic explanation and demonstration of masking. We also cover the various brushes and strokes available within Zbrush 4 before moving on to more in depth subjects in Part Two.”