Skateboard bearings are one of the most important items you should purchase when looking to put together a new deck. Skateboard bearings affect the smoothness of your ride as well as how fast you are able to get going and control your speed. Most skateboard bearings range in ratings from Abec 3 (most basic) all the way up to an Abec 9 (most advanced) rating.
Typically you should go with a minimum of abec 5 rating on any skateboard bearings you are looking at. The difference in ratings between all of the skateboard bearings is the tollerances of the bearings to the casing. The better tolerance (higher abec rating), the better the skateboard bearings will roll and keep speed.
There are a few major components to a typical skateboard bearing. First you have the metal outside housing. and the internal metal housing. All of the ball bearings sit inside of that area. Holing the ball bearings in place is a plastic guard that keeps everything spaced out and even. The plastic guard will keep your bearings from locking up or jamming in the future.The last major part of the skateboard bearings is the plastic dust covers. This keeps dirt and debris from entering the skateboard bearing and causing it to seize or lock up.
Know that more expensive is not always better when choosing a good set of skateboard bearings, so do your research on all of the different brands and choose a good abec rating.