Buying bearings

Abec number
Doesn't mean much; quality of the steel is everything when talking lifespan of bearings. Seal and Lubrication are important to speed.
Seal
Shield with outher ring. High speed, easy maintenace. With self-clinching seal you get a good protection against dirt.
Lubrication
Oil: Highest speed, attracts more dirt, can be rinsed out with water, does not last long.
Grease: lower speed, attracts less dirt, water resistant, lasts much longer.

Notice: Avoid water & sand.
A bearings lifespan will be shortenend drastically when riding /stored in moist surroundings.
 

Vocabulary terms

608: std. bearings 
688: mini-bearing
Ceramics: Caramic bearing made of silicium
RS: One side of the bearing closed, other side with rubber seal.
ZZ: both sides closed with seal and outer ring.
Speed: Use oil by opening one side
Fitness: Oil/grease depending on level of ambition and maintenance. Hockey: Grease
Aggressive: Grease

Other articles:
- Bearings - terms explained
- Bearing cleaning and lubrication