Mafia

The Mafia (also known as Cosa Nostra or "Our Thing"), is a real-life criminal secret society which first developed in the mid-19th century in Sicily. An offshoot emerged on the East Coast of the United States during the late 19th century following waves of immigration. In America, the Mafia often refers to Italian American organized crime in general, rather than just Sicilian American organized crime. Each Mafia family is lead by a Don or Capo Crimine.

There have been many fictitious Mafia families in GTA games, including:

Leone Family - Based in Liberty City, originating from Sicily
Sindacco Family - Based in Las Venturas, and formerly Liberty City
Forelli Family - Based in Liberty City; But may be non-exisitent, after events of GTA 3.
Sicilian Mafia - Shown in Liberty City
Russian Mafia - Shown in San Andreas, appear in league with the Big Smoke and the Ballas, Vagos and Rifas
Trailer Park Mafia - Former redneck gang in Vice City
Vercetti Gang - Most powerful organized crime group in Vice City.