If you're unfamiliar with mecanum wheels, this 48mm Diameter Mecanum Wheel may resemble an everyday chunky wheel Frankenstein'd out of mini hair curler rollers. In reality, it's a very cool, omnidirectional wheel with rollers which are supported by ball-bearings, allowing a four-wheeled robot rober to move in any direction. There are nine rubber beads around the perimeter of the wheel that lets it grip the floor when moving sideways.

These wheels have a little insert that lets them be used by LEGO-compatible cross-bar or low cost TT motors. They simply snap on.

This is the LEFT wheel, you can tell because there's an L embossed on the hub. You will need at least 2 left and 2 right wheels in order to use all of the movement combinations.