Short answers, pulled from the story.
A gang is a group of associates, friends, or family members with defined leadership and internal organization that claims control over territory and engages individually or collectively in illegal, and possibly violent, behavior. The word gang derives from the past participle of Old English gan, meaning to go, and is cognate with Old Norse gangr.
Gang activity in the United States began on the East Coast in 1783 following the American Revolution. The first recognized street gang, the 40 Thieves, formed in New York City in the late 1820s. Chicago alone had over 1,000 gangs during the 1920s as Prohibition drove a new wave of gang growth.
World Bank surveys conducted for the World Development Report 2011 found that unemployment is by far the most common reason people give for joining a gang. Power, respect, money, and protection are also major factors, particularly for at-risk youth who feel ostracized and lack social support.
Gangs include mafias such as the Sicilian Cosa Nostra and Italian-American Mafia, narco organizations or drug cartels like the Sinaloa Cartel and Medellin Cartel, street gangs like the Bloods and Crips, prison gangs like the Mexican Mafia, outlaw motorcycle gangs like the Hells Angels, law enforcement gangs, and vigilante groups.
Initiation varies widely. Many street gangs practice a beat-in or jump-in, where the candidate is beaten for a set period. The Hells Angels require candidates, called hang-arounds, to be observed by veterans for a year or more. The Cosa Nostra uses a ceremony involving oaths and bloodletting, while Triad initiation takes place at an altar dedicated to Guan Yu with incense and animal sacrifice.
The U.S. Department of Justice estimates there are approximately 30,000 gangs with 760,000 members impacting 2,500 communities across the country. In 2006-58 percent of murders in Los Angeles were gang-related. As of April 2011, the National Gang Intelligence Center had identified members of at least 53 gangs affiliated with the U.S. military.