San Andreas Multiplayer (abbreviated to SA:MP) is a third party multiplayer modification for the Grand Theft Auto: San Andreas computer game. San Andreas Multiplayer is developed by a team of dedicated fans of the Grand Theft Auto series of video games, who work on their own spare time developing SA:MP as a hobby.
SA:MP was created by Kye and other developers in the year 2006, these were also the people who had created VC:MP, though SA:MP was considered superior by both the community and the developers due to the high amount of creativity and expansion that could be done in San Andreas rather than Vice City, for example ability to swim in water and a lot of other things which allowed the developers to create an engine which was more appealing that the older one.
There are a wide range of gamemodes on SA:MP, these vary from the basic things such as deathmatches to things such as roleplay and cops and robber based gamemodes. Each SA:MP server has a specific gamemode in which the server is typically focused on, though there are servers which are classified as "Freeroam" which consist of many different activities that can be done.
PAWN is the official language for scripting SA:MP servers, and is used by thousands of people to create their own server. The most commonly used program to script and compile PAWN is called pawno and is available from the official SA-MP website within the server download package. All PAWN functions and a basic scripting tutorial can be found at the official SA-MP wiki here