The majority of skate wheels accept a standard type of ball bearing, that is also found in scooters and roller skates. The bearing dimensions are 8 x 22 x 7mm.
Bearings quality is almost always rated on the abec scale. It goes from 1 to 9, 9 being the best. A good quality bearing will provide less resistance and better performance at high speed. Even if this rating system is supposed to give you a overall idea of the bearing quality, you can’t fully rely on it. Always check the user feedbacks on a product before buying. Some manufacturers managed to get high ratings on bearings that aren't really good.
While the common bearings are using steel, you can also find ceramic bearings that will generally perform better. Ceramic is less prone to heat up while spinning, resulting and almost no material deformation. They are 2 types of ceramic ball bearings: hybrid (where only the balls are ceramic, the inner and outer rings are still metal) and full ceramic. The latest is the best one for performance, although they are a bit more fragile as ceramic has a tendency to break when receiving a shock. Of course they are more expensive than standard steel bearings.
Bearings will always wear out, but if you take care of them, they can last for years. Sealed bearings are really good for skating as they will prevent dust, sand and water from going inside the bearing. It might look like nothing, but if you get a tiny bit of sand in a bearing, you can almost instantaneously tell (when turning the bearing by hand). If a grain of sand goes between a bearing ball and one of the ring, the bearing won't run smooth and will wear out quicker. Of course, I won't tell you to ride only on clean surfaces, that would be stupid. You can clean your bearings, it's not that hard (check that nice guide), your bearings will last longer and you'll get better efficiency out of them.