I have to recommend one of my favorite books and authors; The Guerilla Art Kit by Keri Smith. Her web site is kerismith.com. Please check it out to get a fresh perspective on street art and graffiti.
"Guerilla art is any anonymous work...installed, performed, or attached in public spaces, with the distinct purpose of affecting the world in a creative or thought-provoking way," page 11.
"Guerilla art is for everyone. It engages viewers who might never step foot in a gallery. It is free and accessible," page12.
"There are three possible approaches to guerilla art:
1. beautifying--altering your surroundings
2. questioning--using your voice, challenging the status quo
3. interacting--with the environment or people," page 18
Provide feedback and comments if you can--thank you!