Skateboard bearings are used to attach the wheels of your skateboard to the axle on the truck and allow the wheels to spin. Keeping your bearings clean and well maintained is important for getting the most out of your skateboard.
All skateboard bearings are designed to the industry standard size of 608 with an 8mm bore, a 22mm outer diameter and a 7mm width. This means that they are all the same size, so any bearings should fit any skateboard truck.
Bearings are often rated under the ABEC scale that typically classifies them as precision bearings. They are rated on a scale of 1 to 9 with 9 being the highest rated. Higher ratings generally mean the bearing has been manufactured to a higher precision standard and subsequently is of higher quality and has tighter tolerances. It also means they are more expensive.
Not all brands use the ABEC rating as it was not established with skateboard bearings in mind. ABEC was established for machine bearings, so some skateboard bearing manufacturers have established their own rating systems. For example, Bones use a skate rated system.