Plastic wheel with a cutout specially designed to allow attachment to a Parallax continuous rotation servo. Makes it easy to get your robot rolling. Each wheel comes with a rubber tire that is quite sturdy. Rubberband tires work OK as theyll, but you'll probably want to get 2 or more.
Note: As of Friday, January 15th 2016, they are now shipping these with (1) one thicker, more durable rubber tire instead of (2) two rubberband tires.