Bearings have a couple of measurements that you should be aware of. 8mm, shown with these bearings, tells you the diameter of the inner hole. This diameter matches the diameter of the axle used on the trucks. 8mm axles are by far the most common.
The next number is shown as an abec rating. This number classifies the precision of a bearing when manufactured. Abec1 is considered the lowest rating, and abec9 as the highest. An abec9 bearing will be ground or polished to a higher tolerance and in turn may be faster and last longer. A very precise bearing will have reduced friction, friction causes heat, and heat ultimately will cause bearings to fail.
We always recommend that you clean and lube bearings regularly. A well maintained Abec5 bearing will outlast an Abec7 bearing that is not maintained.