People who kill, rob and threaten others. Gangs can be big ass and rich like the mafia, but they can also be small and/or poor. Most (american) gangs are founded in California by people who didn't have enough money for a good education or didn't manage to get good jobs even with an education(the educated ones can mostly manage more in a gang than a not educated person).
Gangs usually are used by fakers to act tough like 50 cent(the biggest snitch of all times) The Game(one biggass weirdo) Snoop Dogg(a former gangster who claims to be a crip while he is wearing red, yellow, green and colors used by rival gangs)
Gangs are one of the reasons urban dictionary excists cause people do everything, even risking there live, to find out what them gangsters are saying and alotta slang is born that way.
gangster: Yo c wazz da happz you tappin that shit?
5 weeks later.
Person: Were you sayin i had to have sexual intercorse with a poopy?
gangster: What's wrong with you doug?!?!?!
Gangsters do their thang.