Bearings

Bearings. They could be ceramic or metallic. They classify by quality and are marked as ABEC (Annular Bearing Engineering Council). The higher the ABEC number is the more precise is the production. The more accurate production makes the bearing roll faster and with less vibration. The advantage of ceramic bearings is that they have higher firmness, lower  friction coefficient, lower density, lower thermal expansion, endurance, but they have one flaw - fragility which is common to solid materials that’s why they are not recommended for skateboards and longboards that are used for jumping tricks.
Here you can learn the characteristics of Radial run out for every class:

ABEC 1 - 0,008

ABEC 3 - 0,005

ABEC 5 - 0,004

ABEC 7 - 0,003

ABEC 9 - 0,001

High class is not the only thing that points out the quality of the bearings.
Here is the table of ABEC compliance with other standards.