Here’s the Wikipedia entry on “graffiti.” It’s worth reading to getting the full background on the subject, as well as an historical context. And for anyone who asks, “Graffiti” as Wikipedia defines it is …

Graffiti (singular: graffito; the plural is used as a mass noun) is the name for images or lettering scratched, scrawled, painted or marked in any manner on property. Graffiti is sometimes regarded as a form of art and other times regarded as unsightly damage or unwanted vandalism.