In the context of Jamaican popular culture, a sound system is a group of disc jockeys, engineers and MCs playing ska, rocksteady or reggae music. 
The guy holding the microphone is called an DJ-MC and the one puting the turn table needle on the vinyl is called the selector (selecta).

The sound system scene is generally regarded as an important part of Jamaican cultural history and as being responsible for the rise of several modern Jamaican musical genres.

The sound system concept first became popular in the 1950s, in the ghettos of Kingston. DJs would load up a truck with a generator, turntables, and huge speakers and set up street parties. In the beginning, the DJs played American rhythm and blues music, but as time progressed and more local music was created, the sound migrated to a local flavor.

The sound systems were big business, and represented one of the few sure ways to make money in the unstable economy of the area. The promoter (the DJ) would make his profit by charging a minimal admission, and selling food and alcohol.

Competition between these sound systems was fierce, and eventually two DJs emerged as the stars of the scene: Clement ‘Coxsone’ Dodd, and Duke Reid.

The popularity of a sound system was mainly contingent on one thing: having new music. In order to circumvent the release cycle of the American record labels, the two sound system superstars turned to record production. Initially, they produced only singles for their own sound systems, known as “Exclusives” or Dubplates – a limited run of one copy per song.

What began as an attempt to copy the American R&B sound using local musicians evolved into a uniquely Jamaican musical genre: ska. 

Article posted by reggae united world wide/taken from wikipedia. 

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