SAMP means:San Andreas Multiplayer (SA:MP). It is a modification for Grand Theft Auto: San Andreas, PC-Version. This little program turns GTA-SA into a multiplayer game. It allows Players to play over the internet or LAN with other people. Notice: The original Grand Theft Auto: San Andreas game is needed 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 give you the possibility to create your own gamemodes.
For further information see: