Here is an index of sorts for more pages with information that summarize the character and actions of the groups of people that are on the street and in transit across America spreading their names through graffiti. Each distinct group listed below is (or soon will be) a link to a page describing their attributes and activities.
1. Hobos/Freight train riders
2. Hip-hop aficionados
3. Street Gangs - Hispanic, Arian Nation, African-American
4. Political demonstrators
A similar listing (categorization) appears under the heading "Types of Graffiti". The overlap and ambiguity of graffiti is part of the nature of this beast. It is often difficult to pin down graffiti and stick a label on it. This shouldn't be surprising since the writing and the writers are so ephemeral and elusive.