Bearing Standards

You can learn a lot about a bearing just from its part number.
A typical bearing is the 6203ZZ bearing. This can be divided into it's components:

6203ZZ
6 = Type Code
2 = Series
03 = Bore
ZZ = Suffix
The method is followed universally

For the type codes
1 Self-aligning ball bearing
2 Wide self-aligning ball bearing
22 Self-aligning roller bearing
23 Wide self-aligning roller bearing
3 Double-row angular contact ball bearing
4 Double-row ball bearing
5 Thrust ball bearing
6 Single-row deep groove ball bearing
16 Thin section single-row deep groove ball bearing
7 Single-row angular contact bearing.

For an inch bearing, the first digit is R.
R Inch (non-metric) bearing.

Series identify the lightness or the heaviness.
7 Not sure
8 Extra thin section
9 Very thin section
0 Extra light
1 Extra light thrust
2 Light
3 Medium
4 Heavy

Then come the bore sizes in mm
Except for 0 through 3, the bore size is simply five times the third and fourth digits together. 0 through 3, however, are different.
00 10mm
01 12mm
02 15mm
03 17mm

The last suffix is the bearing speciality
Z Single shielded
ZZ Double shielded
RS Single sealed
2RS Double sealed
V Single non-contact seal
VV Double non-contact seal
DDU Double contact seals
NR Snap ring and groove
M Brass cage.

Apart from these there are nomenclatures for different companies that may vary and should be referred to before buying.