Styles of graffiti

Good examples of styles 

http://www.fatcap.com/graffiti-styles.html

Arts or vandalism 

http://www.obsidian.bild-art.de/index.php?option=com_content&view=article&id=238%3Agraffiti-art-or-vandalism&catid=70&Itemid=79&limitstart=1

Different forms and techniques of street art

http://www.graffitistudio.net/different-forms-street-art