Those were suffixes that come after a card's number (in new cards they became prefixes), it is part of the confusing naming scheme that Nvidia uses.
In short, two cards with the same number but different suffix will be different in terms of performance, for example: 8800 ultra is faster than 8800 GTX which is faster than 8800 GT which is faster than 8800 GS.
This is a list of them ordered in descending performance (although there may be some exceptions):
You cant say that 8600 GTS is faster than 8800 GS (it is actually slower), because you have to compare 2 cards with the same number.
Tom has a complete list of graphics cards sorted according to descending performance, youy can look at it:http://www.tomshardware.com/reviews/geforce-310-5970,2491-7.html