There have practically been gangs in the US since the immigration started. This is because of the ethnic and cultural differences, caused by having immigrants from more than one nation. Initially, there were Irish, English, Scottish and German gangs. They were fighting with each others to control districts in the cities, which can be seen in the film Gangs of New York (Great film, by the way). The development from here has mostly involved new immigrants making new gangs for people of their nationality. The gangs probably had much influence on politics, giving hard and soft money to politicians that supported them.
The gangs mostly remained like this until the 1920s - the roaring twenties. During this period selling alcohol was prohibited, which caused organized smuggling, illegal production and bars. These were owned by gangs, although the term "mafia" may be more appropriate now. The mafia largely consisted of Italian and Russian families. Even after the Prohibition was removed, the mafia didn't disappear.
Along with the Civil Rights Movement, African-American gangs developed. Civil rights and organized crime stood quite close, an example of this is the Black Panther Party, which promoted violence and crime along with rights for black people.
Not much has happened since that time; there is an increase in Asian and South-American gangs and there are a few larger gangs that spread across multiple states and/or are not limited to one culture. There is also a separate system of prison gangs, these are largely based on ethnicity. You can see lists of American gangs here.
An interesting thing, is the gang tattoo-aspect. Most gangs have their own tattoo and it was common to tattoo a teardrop for every person you had killed. The picture is related to this, especially Asian and Hispanic gangs are famous for their tattoos. I would really like to make an update on this, but I don't think I will have to opportunity to - I guess this should have been the topic for my senior project.
