"A person, usually a man, who has taken to a life of violent crime. Usually raised in a hostile, deprived environment. They could be hanging with gangs on street corners. They differ from gangsters in the respect that hey are not into organising crime. A thug could graduate into a gangster and may work for one, while still doing spontaneous muggings or dealing.
When in gangs they can be proprietary about their turf."
http://www.urbandictionary.com/define.php?term=thug