Ceramic bearings have the normal steel balls replaced by black ceramic balls.
The ceramic balls are made of solid Silicon Nitride. Ceramic silicon nitride balls are called ceramic but have nothing in common with household dish ceramics. They are called ceramic because it's easier calling them ceramic then Silicon Nitride. The ceramic ball is by far, smoother, harder, lighter and stronger than one made off steel.
The following is a list of benefits of the ceramic Silicon Nitride ball bearing -

• LIGHTER. The ceramic ball is lighter then the steel ball. This means that the ball exerts less force outward against the outer race grove as the bearing spins. This reduction in outward force reduces the friction and rolling resistance. The lighter ball allows the bearing to rev up faster, and uses less energy to maintain it speed.
• HARDER. The ceramic ball is harder then the steel ball. This gives you a bearing that will last up to 10 times longer then the steel ball bearing because it holds it's spherical shape better.
• SMOOTHER. The ceramic ball has smoother surface properties then the steel ball. This means less friction between the ball and bearing races giving you a faster spinning bearing.
• THERMAL. The ceramic ball has better thermal properties then the steel ball. The ceramic ball will not heat up like a steel ball. This helps avoid heat build up in the bearing, a primary culprit in skate and skateboard bearing performance degradation.
• LUBRICATION. The ceramic balls are impervious to oxidation, chemicals, and require essentially no lubricant. The bearings are lubricated with a very, very small amount of Aral Aralub oil, not a gel or grease.

SOLID CERAMIC. The ceramic balls are solid ceramic Silicon Nitride. A cheaper ceramic bearing uses steel balls with a thin ceramic layer.  The elastic and thermal property difference between steel and ceramic cause the ceramic to split and separate from the steel portion of the ball on these less expensive look a likes. Avoid this type of ceramic ball bearing.

Question : What's the difference between the bearing ABEC ratings?
Answer : The term that skaters and skateboarders associated with bearings is "ABEC". ABEC is the acronym for Annular Bearing Engineering Council. ABEC is the standards committee of the Anti-Friction Bearing Manufacturers Association (an industry group) that determines the requirements for each level of bearing precision. The higher the ABEC number, the stricter the manufacturing tolerances and the more precisely the bearing is made. Pleasure Tool bearings are available in ABEC3, ABEC5 and ABEC7.

• ABEC 1 bearing - Inexpensive bearing. Most common bearing with new inexpensive skates and skateboards. They don't provide a frictionless feeling like the other high rated ABEC bearings. Upgrade from an ABEC 1 to a higher rating and you will notice.  We don't sell ABEC 1 bearings.
• ABEC 3 bearing - Common bearing on new "middle of the road" type skates and skateboards. Great for roller hockey and for skaters that need to replace bearings frequently due to the conditions of the skating surface.
• ABEC 5 bearing - An upgrade from most new skates and skateboards. Great for the recreational skater or skateboarder that wants that frictionless feeling. Most common upgrade due to the fact that the price is not much more then the ABEC 3 bearing. ABEC 5 bearings cruise very nicely and have a nice frictionless feeling. 
• ABEC 7 bearing - A racer's bearing. ABEC 7 bearings are used by serious skaters and skateboarders that want the best. The manufacturing of this bearing is difficult and costly. The tolerances are very tight and they have virtually no friction under the load of your weight. They are truly a nice bearing to experience for any kind of skater or skateboarder. We sell a great number of ABEC 7 bearings due to our competitive pricing.