Loads of street gangs(or well, we should call them sets) have a certain structure.

My guess is, the main difference between the mob and the gang is that people who are in the mob are men, whereas a lot of gangs are made up of, well, kids in some cases. I understand that a lot of mob guys "grew up in the life", but in gangs you've got kids being jumped in at the age of 9-10-11 and are well into drugs and guns by the time they hit puberty. Not always, but quite often.