To give the object certain quality or texture. 

MASK: ctrl, to select the area that is certain type and drag the ball into it.

UV: necessary to having to quickly draw in 2D surface, the UV is the important part to give the designer the clue of parts are.

Brush: The brushes in pt are use as texture giver, although it is also applicable in Z-Brush, But Pt give quick and more texture and color than gray model in Z-brush. But it is hard to model the object in PT.

Texture balls: Are also used as brush, some of them do not have color change.