UV Mapping is the process by which a two-dimensional image is wrapped onto a three-dimensional object. UV coordinates (or UVs for short) have to be assigned to a model before the image can be displayed correctly.
Think of it as a bit like peeling an orange. You can cut the peel off the orange and lay it out flat – that’s like your 2D image; and you could wrap the peel back around the orange and it would fit perfectly – that’s like the 2D image mapped to the 3D object. The UVs are needed so that the 3D program knows which bit of the image goes exactly where on the 3D object. There are many ways of peeling an orange, and there are many ways of assigning UV coordinates.
ZBrush has several ways for assigning UVs: