Bearings are rated according to the precision of the manufacturing process.  The scale for rating is known as the ABEC scale (Annular Bearing Engineering Council).  ABEC ratings are 1,3,5,7 and 9.  The higher numbers designate a higher-precision bearing and will cost more.

One additonal aspect of the higher ABEC-rated bearings is that they tend to be serviceable, meaning they can be disassembled for cleaning and relubrication.

There are also differences in the materials that the bearings are made from.  Most ball bearings are made from chromium steel while the cage and shields may be made of other materials such as steel or nylon.  Ceramic ball bearings are available also, but at a much higher price than chromium steel.