Rear wheels are made of primarily two rubber compounds: Natural and Wonder. Natural sometimes referred to as "Fish" for its unique smell are softer tires which are best suited for scale and flat track racing where the cars lack the aerodynamic downforce prevalent in wing car racing. With such they wear faster and provide much more traction (bite). Rubber also comes in Foam which offers less traction, Some rubber can also be treated for added hardness.
Wonder rubber is a harder compound and provides less tracktion and longer wear. They work best for wing car racing where the aerodynamic downforce is greater due to the wings making traction less of an issue.
All tires come in pairs and are available for 2mm, 3/32" and 1/8" axles.