What is graffiti?

Before I can start writing about graffiti I have to explain what graffiti is. 
Therefore I found a nice definition at "www.wikipedia.org" because my English skills aren't good enough to explane it by myself.

"Graffiti are writing or drawings that have been scribbled, scratched, or sprayed illicitly on a wall or other surface in a public place. Graffiti ranges from simple written words to elaborate wall paintings, and it has existed since ancient times, with examples dating back to Ancient Egypt, Ancient Greece, and the Roman Empire.

In modern times, paint, particularly spray paint, and marker pens have become the most commonly used graffiti materials. In most countries, marking or painting property without the property owner's consent is considered defacement and vandalism, which is a punishable crime."

Found at wikipedia.org