You got all different servers;
- Roleplay (RP) which is basically pretending to be the character you see on the screen. It's more fun than it sounds like.
- Deathmatch (DM) which is self explanatory, just killing each other.
- Stunt, which is riding around on bikes, cars and other vehicles and jumping over self build ramps.
- Freeroam, which differs per server but in most of the servers it's DM combined with Stunting.
It's quite a fun game and any PC can run it, let me know if you're planning on playing or if you got any questions.
You can download the multiplayer version here; SA-MP San Andreas Multiplayer mod for Grand Theft Auto (GTA SA) you will need the regular version of San Andreas fully installed first.