To create a base mesh for your sculpting in ZBrush you can use ZSpheres, ZSketch, ShadowBox, Dynamesh or import a model from another 3D package. Another solution is to do a Remesh of a model or a set of different objects. The Remesh method creates new geometry with a new topology, but as much as possible that topology is based off the original model(s).
ZBrush uses its voxel-based Unified Skin technology to create the model. The purpose of these tools is to create a new base mesh that you can then sculpt via traditional methods to further refine the shape.
One or more SubTools can be remeshed and modifiers applied to perform Boolean operations (Add, Subtract, Intersect). Remesh can also make use of Symmetry for truly spectacular – and sometimes startling – results.