San Andreas Multiplayer (SA:MP) is a modification for Grand Theft Auto: San Andreas which turns it into a multiplayer game. You can play over the internet (or LAN) with up to 500 other people. You need the original Grand Theft Auto: San Andreas PC game to play San Andreas Multiplayer.
In SA:MP you are able to do almost anything that you can do in single player GTA. SA:MP also features the ability to create your own scripted game modes. There are many game modes that feature a series of different and fun things to do. Many of which are not possible in single player.
To go to official SA-MP Wiki page, just click the following image: