What are the different grades of ball bearings?

From: Bill Codding , Harry Phinney 

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 hardened.