The word "gang" derives from the past participle of Old English gan, meaning "to go". It is cognate with Old Norse gangster, meaning "journey." It typically means a group of people, and may have neutral, positive or negative connotations depending on usage.

In discussing the banditry in American history Barrington Moore, Jr. suggests that gangsterism as a "form of self-help which victimizes others" may appear in societies which lack strong "forces of law and order"; he characterizes European feudalism as "mainly gangsterism that had become society itself and acquired respectability through the notions of chivalry".

A wide variety of gangs, such as the Order of Assassins, the Damned Crew, Adam the Leper's gang, Penny Mobs, Indian Thugs, Chinese Triads, Snakehead, Japanese Yakuza, Irish mob, Pancho Villa's Villistas, Dead Rabbits, American Old West outlaw gangs, Bowery Boys, Chasers, the Italian Mafia, Jewish mafia, and Russian mafia crime families have existed for centuries. According to some estimates the Thuggee gangs in India murdered 1 million people between 1740 and 1840.

The 17th century saw London "terrorized by a series of organized gangs", some of them known as the Mims, Hectors, Bugles, and Dead Boys. These gangs often came into conflict with each other. Members dressed in the following way: "with colored ribbons to distinguish the different factions."

Chicago had over 1,000 gangs in the 1920s. These early gangs had reputations for many criminal activities, but in most countries could not profit from drug trafficking prior to drugs being made illegal by laws such as the 1912 International Opium Convention and the 1919 Volstead Act. Gang involvement in drug trafficking increased during the 1970s and 1980s, but some gangs continue to have minimal involvement in the trade.

In the United States, the history of gangs began on the East Coast in 1783 following the American Revolution. The emergence of the gangs was largely attributed to the vast rural population immigration to the urban areas. The first street-gang in the United States, the 40 Thieves, began around the late 1820s in New York City. The gangs in Washington D.C. had control of what is now Federal Triangle, in a region then known as Murder Bay.

In 2007, there were approximately 785,000 active street gang members in the United States, according to the National Youth Gang Center. In 2011, the National Gang Intelligence Center of the Federal Bureau of Investigation asserted that "There are approximately 1.4 million active street, prison, and outlaw gang members comprising more than 33,500 gangs in the United States." Approximately 230,000 gang members were in U.S. prisons or jails in 2011.

According to the Chicago Crime Commission publication, "The Gang Book 2012", Chicago has the highest number of gang members of any city in the United States: 150,000 members. Traditionally Los Angeles County has been considered the Gang Capital of America, with an estimated 120,000 (41,000 in the City) gang members.

There were at least 30,000 gangs and 800,000 gang members active across the USA in 2007. About 900,000 gang members lived "within local communities across the country," and about 147,000 were in U.S. prisons or jails in 2009. By 1999, Hispanics accounted for 47% of all gang members, Blacks 31%, Whites 13%, and Asians 7%.

In December 13, 2009, The New York Times published an article about growing gang violence on the Pine Ridge Indian Reservation and estimated that there were 39 gangs with 5,000 members on that reservation alone.

There are between 25,000 and 50,000 gang members in Central America's El Salvador.

More than 1,800 gangs were known to be operating in the UK in 2011.

The FBI estimates that the four Italian organized crime groups active in the United States have 25,000 members in total.

The Russian, Chechen, Azerbaijani, Ukrainian, Georgian, Armenian, and other former Soviet organized crime groups or "Bratvas" have many members and associates affiliated with their various sorts of organized crime, but no statistics are available.

The Yakuza are among one of the largest criminal organizations in the world. As of 2005, there are some 102,400 known members in Japan.

Hong Kong's Triads include up to 160,000 members in the 21st century. It was estimated that in the 1950s, there were 300,000 Triad members in Hong Kong.

Perhaps one of the most infamous criminal gangs are the Sicilian Cosa Nostra and the Italian-American Mafia. The Neapolitan Camorra, the Calabrian 'Ndrangheta and the Apulian Sacra Corona Unita are similar Italian organized gangs.

Other criminal gangs include the Russian mafia, Mexican, Colombian Drug Cartels, the Aryan Brotherhood, the Mexican Mafia, the Texas Syndicate, the Black Guerrilla Family, the Nuestra Familia, the Mara Salvatrucha, the Primeiro Comando da Capital, the Irish Mob, the Puerto Rican Mafia, Nuestra familia, the Chinese Triads, the Japanese Yakuza, the Jamaican-British Yardies, the Haitian gang Zoe Pound, and other crime syndicates.

On a lower level in the hierarchy of criminal gangs are street gangs in the United States (mostly branches of larger criminal gangs). Examples include:

• Black gangs like the Bloods and the Crips
• National origin and/or racial gangs like the Trinitario, Sureños, Tiny Rascal Gang, Asian Boyz, Wa Ching, The Latin Kings, The Hammerskins, Nazi Lowriders and Blood & Honour.
• Biker gangs such as the Hells Angels, the Pagans, the Outlaws, and the Bandidos, known as the "Big Four".

Many types of gangs make up the general structure of an organized group.

There are street gangs, with members of similar background and motivations. The term "street gang" is commonly used interchangeably with "youth gang", referring to neighborhood or street-based youth groups that meet "gang" criteria. Miller (1992) defines a street gang as "a self-formed association of peers, united by mutual interests, with identifiable leadership and internal organization, who act collectively or as individuals to achieve specific purposes, including the conduct of illegal activity and control of a particular territory, facility, or enterprise."

Understanding the structure of gangs is a critical skill to defining the types of strategies that are most effective with dealing with them, from the at-risk youth to the gang leaders. Not all individuals who display the outward signs of gang membership are actually involved in criminal activities. An individual's age, physical structure, ability to fight, willingness to commit violence, and arrest record are often principal factors in determining where an individual stands in the gang hierarchy; now money derived from criminal activity and ability to provide for the gang also impacts the individual's status within the gang. The structure of gangs varies depending primarily on size, which can range from five or ten to thousands. Many of the larger gangs break up into smaller groups, cliques or sub-sets. The cliques typically bring more territory to a gang as they expand and recruit new members. Most gangs operate informally with leadership falling to whomever takes control; others have distinct leadership and are highly structured, which resembles more or less a business or corporation.

Prison gangs are groups in prison or correctional institution for mutual protection and advancement. Prison gangs often have several "affiliates" or "chapters" in different state prison systems that branch out due to the movement or transfer of their members. The 2005 study neither War nor Peace: International Comparisons of Children and Youth in Organized Armed Violence studied ten cities worldwide and found that in eight of them, "street gangs had strong links to prison gangs". According to criminal justice professor John Hagedorn, many of the biggest gangs from Chicago originated from prisons. From the St. Charles Illinois Youth Center originated the Conservative Vice Lords and Blackstone Rangers. Although the majority of gang leaders from Chicago are now incarcerated, most of those leaders continue to manage their gangs from within prison.

Criminal gangs may function both inside and outside of prison, such as the Nuestra Familia, Mexican Mafia, Folk Nation, and the Brazilian PCC. During the 1970s, prison gangs in Cape Town, South Africa began recruiting street gang members from outside and helped increase associations between prison and street gangs. In the USA, the prison gang the Aryan Brotherhood is involved in organized crime outside of prison.