What are bearings?
Bearings allow your wheels to spin freely.  Each wheel on your skate contains two bearings and one spacer in the center.

How many do I need?
When buying bearings, remember to buy two for every wheel that is on your skate.  If your skates have 8 wheels total, you'll need 16 bearings.

Which ones should I buy?
Generally speaking the more money you spend on bearings, the less friction your wheels will have while you're skating.  In other words, you'll be able to skate faster and maintain you speed easier.

Do I need spacers?
Yes.  Before you throw your old wheels and bearings out, make sure to keep the spacers.  Spacers are found in-between the two bearings in the centre of your wheel.  Without spacers, your wheels won't roll properly.  If you need spacers you can buy these 8mm Standard Spacers which will work with most wheels and skates.