I am looking to buy some sanyo NCR18650GA batteries but then I saw the NCR18650B.
What does GA and B mean?
If you win the batteries in a giveaway, they're GA.
If you buy them, they're B. 
? All my sanyo 18650s I bought from illumn are NCR18650GA - I certinally did not win them
I know...
I'm kidding. 
Dammit lol
As long I can remember.., but I could be wrong , The "GA" has less capacity , but bigger current capabilities ( maybe 10 Amps ) , and "B" are viceversa ( 5Amps)...
I dare you , change my mind !
The GA has higher current and capacity 10A 3500mAh.
Ummm…. No.
Maybe the OP can clarify, I don’t think they were asking the difference in the cells, but asking in general what do the mean, or represent. The answer to which is, what ever the manufacturer wants. They simply mean it is in someway different, which could be any number of things from testing protocol, vent cap, precise chemistry, or just the wrapper 
The GA is newer and better on almost everything but price and availability. The B is an older model but it’s still in use. The voltage under load is quite a bit higher for the GA.
From HKJ’s tests the capacity to 3.0V is better for the GA, except at very low current (0.2A) where it’s almost the same. The difference at 3.0A (a regulated flashlight) seems about 125mAh.
Well I’m new here but not to Google, for example:
Panasonic NCR18650B
In this pdf you will find ” NNP technology ” , Google that , will get you here:
http://www.maxcapacity.co/advanced-nnp-technology-from-panasonic/
In here you will find ” advanced NNP technology from Panasonic helps prevent negative electrode deforming with repeated charging. ”
So this is just 1/2 of the meaning of ” B “.
I have the exact Questions you do, but some of this refers to manufacturing jargon and its not helping us very much.
So I guess maybe check here for some reference to battery testing for comparison
http://lygte-info.dk/review/batteries2012/Common18650comparator.php
Remember I’m new to this , so let me know if i screwed up somewhere….