Graffiti is a commonly misunderstood form of illegal art. Graffiti is done on walls, buses, trains, and other forms of private and public property, often without permission (thus the 'illegal' part). Ink, paint, scratches, and other ways can be used to do graffiti; spray paint cans with special tips being the favored way, since they are easily carried and do not require brushes and other equipment. Note that not all things sprayed/written on a wall are graffiti! Gang markings, crude bathroom writings, and other less artistic forms of wall-markings are NOT graffiti.
Graffiti (also called Graf) is a form of self expression, and while it is illegal, is NOT vandalism.
Closely related to people who tag
Man, I saw some awesome graf down by Ninth Street yesterday.