Gangs are organizations of the street composed of either 1. the socially excluded or 2. alienated, demoralized, or bigoted elements of a dominant racial, ethnic, or religious group.

What is important is that the gang is not a stable form, but can, and has, changed its shape and functions over time. While most gangs begin as unsupervised adolescent peer groups and remain so, some institutionalize in barrios, favelas, ghettoes, and prisons. Often these institutionalized gangs become business enterprises within the informal economy and a few are linked to international criminal cartels. Others institutionalize as violent supporters of dominant groups and may devolve from political or conventional organizations. Most gangs are characterized by a racialized or ethno-religious identity as well as being influenced by global culture. Gangs have variable ties to conventional institutions and, in given conditions, assume social, economic, political, cultural, religious, or military roles.

When we look around the world, as in A World of Gangs we find a world of different shapes of gangs and armed groups. While we can separate a gang from an ethnic militia in theory, it is not so easy in practice. Gangs sometimes evolve into political, ethnic, or even religious forms. At times political or military forms devolve into gangs, concerned mainly with criminal venture.