Standard skateboard wheel bearings are classified as a 608. The 608 is merely a bearing family classification system, meaning it is a 600 series bearing with an 8mm inside hole which is the part that fits over your truck axle. A 608 bearing is used for many applications including rollerblades, vacuum sweepers, atm machines.
There are many brands of skateboard bearings on the market today.  Powell Bones,  Lucky,  Shorty's Black Panthers,  Diamond,  Pig,  Tensor,  Element, 

What is ABEC?
ABEC is a rating system used by bearing manufacturers to determine the precision to which a bearing will be manufactured to get the specific rating.
ABEC stands for ABEC stands for Annular Bearing Engineers Committee. This committee works to determine the standards for bearings for the Anti-Friction Bearing Manufacturers Association. What it boils down to is that a bearing must meet certain specs to get the rating of ABEC 1, ABEC 3, ABEC 5, or ABEC 7.
However, you must know that the ABEC committee rates bearings for use at 32,000 RPM which converts to over 300 mph. I do not think you will ever go that fast on your skateboard.

What are the shields
The shield is the outside of the bearing that you can see, it is either removable or non-removable. Most skateboard bearings use removable shields, like Bones bearings . Usually the shield is metal covered in neoprene. This is probably the most popular choice since you can pop off the shield clean the bearing, put in a couple drops of lube and pop the shield back on, usually without removing the bearing from the wheel. Non-removable metal shields are used by Lucky bearings and require a little more work to clean.

Skateboard Bearing FAQ
Everyday we get the same questions from our customers on skate bearings
1. What is the fastest bearing - Well a bearing is part of the speed equation. If you have tiny little beat up wheels with some Bones Swiss you will not go that fast. The wheel has alot to do with speed and cored wheels will offer the highest level of performance for your bearings. Ricta,  Hubba,  and Spitfire  all make some very good cored skateboard wheels. A bigger wheel will also go faster than a small wheel. A big wheel being 54mm - 58mm but be cautioned the bigger the wheel the farther your tail has to go to hit the ground which makes every trick that much harder. Most street skaters use a 49mm - 53mm wheel. Ramp or skatepark riders will use a 54mm - 60mm. Here is a little saying we have...your skateboard can only go as fast as you push it, but how long it goes that fast it what makes a good bearing.
2. What is the best bearing? - Empiresk8 usually does not recommend products, but we will tell you that we sell way, way more Bones Reds, Bones Swiss, and Bones Super Swiss 6 Bearings than any other bearing. We are not saying that they are the best but people buy Bones bearings again and again. We have customers that have had their Bones Swiss bearings for 3 - 4 years and they still spin fine.