Among Latino gangs, graffiti is more commonly known as "placas" and sometimes as "tagging." On the other hand, not all "taggers" are gang members. Some taggers are clearly "independent operators" separate from the gang although generally on friendly terms with the members. These taggers seem to enjoy vandalizing property with their tags almost like Tom cats spraying territory for others to note and beware. However, some taggers do not commit any other kinds of crimes.
Gang graffiti appeared everywhere in the neighborhoods observed in this research--on fences, walls, businesses, street signs, on private property, on buses, freeway overpasses, and billboards.
Entire inquiries and analysis of graffiti and their significance have been done and probably should continue. An interesting linguistic corpus is right there ripe for the plucking.