All skateboard bearings are the same size, and fit all sizes/types of wheels. Bearings allow the wheel to spin freely around the axle of the truck. You need 2 bearings per wheel, so for a set of wheels (4 wheels), you need 8 bearings total (a set of bearings always brings 8, unless sold individually). The main differences between brands/models is quality as determined by materials, balls per bearing, lubrication, seal, etc. The better bearings are more durable, and give you a smoother and faster ride.