Your bearings are inside little metal rings that fit inside your skateboard wheels. There's only one way to rate bearings at the moment, and it doesn't work well with [skate bearings] . The rating is called ABEC and goes from 1 to 9, but only odd numbers. Unfortunately it was originally developed to rate the bearings in machines, not on skateboards (for more, you can read "What does ABEC mean?".

Therefore, the ABEC rating only rates the precision of a bearing. Plus, the more precise he bearing, the weaker they usually are. Skateboarders take their bearings and abuse them, as normal skateboarding does. Skateboarders want bearings that are both precise and durable, so the ideal ABEC rating for a skateboard is 3 or 5. Smooth enough, but not going to break when you jump on your board. Some skate bearings don't even bother with the ABEC rating system. The best thing to do is try some out, ask your friends, or ask the guy behind the counter at the skate shop.

A Shielded Bearing has a metal cover on the Outside of the Bearing and is Attached to the bearing either by a Clip (Serviceable) or is Pressed into the Bearing called a Stake Shield (once removed it cannot be re-installed) and is most Common.  Shields do not keep the elements out as well as a Seal but there is no drag on the Bearing whatsoever making it a Faster Bearing as there is no Contact with the Shield. It is also easier to maintain a Shielded Bearing as you can lubricate the skf Bearings without having to remove them.

Skateboarders take their bearings and abuse them, as normal skateboarding does. Skateboarders want bearings that are both precise and durable, so the ideal ABEC rating for a skateboard is 3 or 5. Smooth enough, but not going to break when you jump on your board. Some ina bearings don't even bother with the ABEC rating system. 

SN516

SN517

SN518