About Z2 , all I can tell is that the "tweak" brush has been replaced by "Move" brush in Z3 . I've never used Z2 , I can't really help , here you can fins something more about the brushes in Z3:
http://www.zbrush.info/docs/index.php/Brush_Types

Pixologic.com and zbrushcentral.com are two sites where you can find TONS of "how to" , and a forum with very specific questions . Just start with a basic figure and play with the brushes so you can see how they act ( in Zbrush , "tool" means your mesh and "brush" is the tool you use to sculpt your mesh ...).

The basis are the same , whether you use Z2 or Z3.

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