This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Bill Codding <peda@simplicity.Stanford.EDU>,
Harry Phinney <firstname.lastname@example.org>
Following is a description of the different grades of ball bearings.
The grade specifies the sphericity of the balls in millionths of an inch.
Thus, grade 25 are round to 25/10^6, while grade 1000 are good to 1/1000
(i.e. not all that round, but probably good enough for our uses).
Grade 25: the highest quality normally available, aka
"Campagnolo quality": hardened all the way through, best
alloys, coatings, roundness, and durability. Evidently,
a recent bottom-bracket overhaul article in "Bicycling Plus
Mountain Bike" magazine recommended these. Campy's tech reps
claim that the bearings in a set (usually in a little paper bag)
are matched. One should not mix bearings from different sets.
Grade 200: mid-range
Grade 1000: seems to be the lowest, may only be surface
Good sources for ball bearings:
Your local bike shop (make sure you're getting the grade you want)
Bike Parts Pacific
Bike Nashbar 1-800-NASHBAR ($1-$3 per 100 Grade 25)
The Third Hand 1-916-926-2600 ($4-$7 per 100 Grade 25)