ZBrush is a digital sculpting tool that combines 3D/2.5D modeling, texturing andpainting. It uses a proprietary "pixol" technology which stores lighting, color, material, and depth information for all objects on the screen. The main difference between ZBrush and more traditional modeling packages is that it is more akin to sculpting.
ZBrush is most known for being able to sculpt medium to high frequency details that were traditionally painted in bump maps. The resulting mesh details can then be exported as normal maps to be used on a low poly version of that same model. They can also be exported as a displacement map, although in that case the lower poly version generally requires more resolution. Or, once completed, the 3D model can be projected to the background, becoming a 2.5D image (upon which further effects can be applied). Work can then begin on another 3D model which can be used in the same scene. This feature lets users work with extremely complicated scenes without heavy processor overhead.