Bearings sit in the wheel hub and are what allows the wheels to roll in the skates. Each wheel has two bearings in it, for a set of quad skates you need 16 bearings total, 8 for each skate, two for each of the four wheels.
In the past bearings were always rated by an ABEC rating scale. Now, most bearings are considered above the ABEC rating scale. This results in many bearings do not have an official rating.
ABEC ratings were designed for high speed applications like high speed motors and precision measuring instruments. As a result in roller skating this does play an important role.
Bearings do NOT make you “go faster”, they do affect your roll out. The roll out is how long your wheels will keep spinning after you push off. A better bearing will keep you rolling longer, so you can put in less effort to cover the same amount of space.
In the past bearings were actually put in by the skater with looseballs, this is very rare now. All bearings come put together.
The parts pictured are:
Shield: the shield which protects the contents of the bearing from acquiring debris. Some bearings will have a shield on each side of the bearing, some will only have one side closed with a shield. If your bearings only have one shield, always face the bearing where the open side is pointed to the inside of the wheel to help keep them as clean as possible. There are different versions of shields as well. Some are sealed and made of metal so that you cannot open the bearing after purchase. This can make it difficult to maintain them and those are usually only found on lower priced bearings that are not intended for long term use. Some bearings will have a metal shield on just one side so that they can be cleaned or a removable metal shield on one or both sides. The down fall with metal shields is they can get bent when maintaining your skates, once that shield bends in and gets in the path of the balls it can slow down the roll of your wheels. Most have a removable nylon shield that you can take out if you are doing maintenance on the bearings. Ideally a nylon shield on each side of the bearing will keep it the cleanest and make it the easiest to maintain.
Inner race, this is the only part that comes in more than one size, 7mm or 8mm. What size you need is determined by what size axle you will be putting the bearing/wheel on. Most axles made recently will be 8mm, with older versions being primarily 7mm.
Balls, these are exactly what they sound like, that is where the magic happens.
Retainer, this keeps the balls from coming out of the bearing if the shield is removed. Outer race, this is the outside of the bearing and what is actually making contact with the wheel once installed.
What size bearing do I need? This is decided by what size axle your skate has. If you are not sure, there is a super awesome cheat. If you take a number 2 pencil and put it through the inner race (the inside hole) if it fits then you have an 8mm bearing, if it does not fit, you have a 7mm.