A gang is an association or group of criminals. Often, members of one gang will fight violently with another gang.
People join gangs for the sense of belonging, as well as for protection by other members. A gang usually claims an area as its territory and defends it against other gangs or unaffiliated criminals. You can also use gang for a group of friends or workers: "A whole gang of us went to the movies together." The Old English root, gang, means "journey." Today's definition comes from the idea of "a band of people traveling together."