Skate bearings are small donut shaped casings which contain tiny metal balls.  The casings can have one or both sides sealed.  A precision bearing has both sides sealed and the balls are packed in grease inside.  These require little maintenance and may last the life of the skates.
 

Semi sealed bearings are closed on the outer side while the inner side exposes the metal balls to view.  These bearings have to be lubricated regularly and do not offer the longevity or quality of precision bearings.

Many bearings are rated on what is known as the ABEC (Annular Bearing Engineering Council) scale which grades on a scale from 1-9 (odd numbers only).  The higher the rating, the better the quality of bearing, the smoother the roll and, as to be expected, the higher the price.  ABEC1's are good for beginner skaters but as one improves they will want better quality and the standard for recreational skaters is the ABEC3 grade.  Higher grades are primarily used by speed skaters.
 

Not all skate bearing manufacturers, however, utilize the ABEC rating system. , states that the ABEC system was developed to grade bearings used in electric motors and not those used in skate wheels.  They claim their bearings are specifically made for, tested and approved by skaters which they believe makes their bearings a better choice.  Individuals will have to make a decision for themselves on which brand of bearing they prefer for their style of skate.