The 2.5D concept. ZBrush combines 2D and 3D. When you are doing basic sculpting and movement you are in 3D but when you add detail and color with projection Master you are in 2D. This is where 2.5D came from. ZBrush and the use of the Pixol rather than the Pixel
• This is from the ZBrush Practical guide

Table of Contents
1. Important settings for working in ZBrush
2. Importing an object from Max and Maya and saving your ZBrush file
3. Morph targets, movement, subdivisions
4. Hiding or showing polygons
5. General sculpting, different brushes
6. Adding detail with projection master, or the stencil tool
7. Adding colors to your object.
8. Normal maps and applying them in Max or Maya
9. Applying your texture with the normal Map in Max
10. My most used hotkeys
11. Pixologic hotkey List
12. A list of my favorite anatomy books and references

Download pdf ZBrush 2.0 Tutorial