There are literally hundreds of different bearings out there that you can buy. The unfortunate thing is that the vast majority of them are glorified washing machine bearings with fancy packaging and big advertising campaigns.

Ball Bearings
The balls in a bearing are contained by delrin crowns that allow the bearing casing to spin around them.
Most bearings contain 7 ball bearings, however the Bones Super Swiss Six Bearings contain 6 bigger balls for increased strength and speed.

ABEC
Stands for Annual Bearing Evaluation Committee. Ratings follow numerical values of 1,3,5,7,9 and are precision ratings i.ex. as to how well a bearing performs under high speeds in a straight line.

Considering the fact that the ABEC rating is applied for industrial machine operations and purposes, ABEC means very little in skateboarding. The fact is, Skateboarding introduces forces and strains that a bearing was never intended to handle. Side loads and high energy impact can break down a bearing at any rating level. So the components quality is the key issue to a good skate bearing.

Here are some more facts
Skateboard bearings allow for the rolling motion of a wheel on it's axle. They consist of 6, 7 or 8 balls enclosed in races between two shields encased in a disklike body. 2 bearings, 1 on each side, are inserted into every wheel, totalling 8 bearings necessary for a complete skateboard.
Industry standard skateboard bearings are of one universal size (type: 608), except for Element Micro Bearings (type: 688) which can be adapted to fit their respective Micro-Core wheels.

Ball retainers
also sometimes called cages or crowns, hold and seperate the individual balls within a bearing. By keeping the ball bearings lubricated and clean within their retrainers, you can lengthen the life and strengthen the performance of your bearings. Cheap bearings use metal like brass or other soft metals for the retrainer. Those are likely easy to corrode, once the lubricant is away, or water found its way into the bearing. This causes the end of life cycle in very short time, as the bearing usally stucks and stop.

Delrin Crown are made out of very durable and heat resistant Nylon and are used mainly in good bearings. They withstand most impact, are unable to corrode and specially curved, to keep the lubricant arround the balls.

Shield
of some bearings (including Rockers, Bones, etc...) are removable to allow for cleaning and lubrication. Others are punched in, and are non serviceable at all.

The bearing shield is essentially the side of the bearing, which keeps dirt away from getting inside. Often there is a C clip to keep the shields in place, in case you have metal shields (one side Z, both sides ZZ).
As well most good bearings use rubber shields (RS), which are easy to pop out with a pin or sharp thin knife. Those rubber shields are equipped with a labyrinth design seal, to prevent most dirt to get in.

The C clip
is a mechanism for locking bearing shields in place. It's basically a thin C-shaped wire that fits tightly into a groove around the outside perimeter of the bearing shield to hold it in place against the bearing casing. Not all bearings have them, but those that do can typically be taken apart for cleaning and maintainence.

Lubricant
is the juice that keeps your bearings alive, quick and quiet. There are three different types: grease, oil and gel.
Grease is usually inside old fashioned or cheap bearings, with metal crowns. Making them turning sticky in the first time, as the grease has to distribute. Advantage it keeps dirt mostly away from the balls.
Oil is used only by speed freaks with high bearings, as it got the lowest friction and leaks out once its hot. As well it got the highest maintainance rate.
Gel is a synthetic blend of low viscosity, high speed oils that minimize friction within the bearing and allow them stay spinning smooth.

New bearings are already lubricated with gels but due to the abuse we dish out they do get dirty after a while. If you don't want to buy new ones, clean them up and try squirtin' some Bones Speed Cream, Reflex Bearing Lubricant or FKD Bearing Lubricant in there to keep them rolling like new. It's a good idea to maintain your bearings on a monthly basis.