The USK rating is independant from the other ratings such as PEGI. There are different levels like USK 0, 6, 12, 16 and 18. You must have an USK rating here to be allowed to sell your game in public. Also important to know that the USK system is a self-regulatory one, so the USK will tell the publisher what can be in a game to get one of the ratings. Today, most stuff passes without any cuts with a USK 16 or 18 rating. But a little bit in the past it was different, lots of games had to tone down on violence to even get an USK 18 rating.
Also since USK 18 games didn't sell well, since games where something seen mostly for teens, in the publishers oppinion at least, they made further cuts themselves (the devs, publisher) to get an USK 16 rating, to broaden their target audience.
Also having an USK 16 rating doesn't mean that it's cut, there are lot of examples of uncut USK 16 games.
Anyway the differences between the USK 16 and USK 18 version of San Andreas is that it was first rated USK 16 and after the infamous Hot Coffee Mod showed up, it was pulled from the stores and rerated as USK 18, there's no further difference between them two.
Furthermore for most older games there are uncut patches or even older ones you just had to change the language or edit some config file. For newer games it's not so easy, looking at you Bethesda and Wolfenstein.