Omni wheels or polywheels are wheels that are composed of usually small rollers that are arranged around the circumference of the wheels and can rotate in a direction perpendicular to that of the wheel as a whole.
The omni wheel as a whole can roll in two directions: forwards and backwards (north and south); the rollers on the wheel also allow the wheel to roll from side to side (east and west). Because the omni wheel can roll from side to side it minimizes the friction associated with turning when using a tank drive or other similar drives.
Omni wheels allow for omni-directional motion in various configurations. The Kiwi drive is one such configuration that uses omni wheels positioned at 60 degree angles from each other to achieve motion in all directions.