Gangs as most people think of them probably began to be recognized by the general public around the nation with the birth of the Los Angelesgangs in the early seventies.
If a gang member witnesses a fellow member failing to dis a rival gang through hand signs, graffiti, or a simple "mad dog" or stare-down, they can issue a "violation" to their fellow posse member and he/she can actually be "beaten down" by their own gang as punishment.
Gangs are certainly a community problem, and the community must galvanize to properly respond by dealing with those already involved and offering alternatives to those who accept.