Tread design involves both the selection of the elastomer (rubber) and the pattern of the rubber surface . Some examples are illustrated below.

The road tires shown to the left have tread designs that remove water to the side, and also a relatively smooth tread to minimize rolling resistance. The lower left photograph shows a mountain bike tire (Michelin Chamonix) that will provide a good trail grip even with a wet, unpaved surface. The lower right picture is designed for a muddy trail (ExperTraction)

Sloane, "Sloane's Complete Book of Bicycling,"
Simon & Schuster (1995)