Graffiti.

Graffiti is writing or drawings that are drawn, painted or spray painted, sometimes scratched and scribbled on walls, buildings or any other surface in public places. A piece of graffiti art usually consists of written words or can elaborate to huge wall paintings. Materials that are commonly used to make graffiti markings are paint, spray paint and marker pens.
Graffiti has been around for many years dating back to the ancient times with examples from Ancient Greece and the Roman Empire. In most countries making marks upon a wall or painting on property without the property owners consent is considered defacement and vandalism, which is a punishable crime. Controversies that surround graffiti continue to create disagreement amongst city officials/law enforcement and writers who wish to display and appreciate work in public locations. There are many different types of graffiti and it is a rapidly developing art form whose value is highly contested, reviled by many authorities while also subject to protection, sometimes within the same jurisdiction.
Graffiti is about expressing underlying social and political messages and a whole genre or artistic expression is based upon spray paint graffiti styles. Within Hip Hop culture, graffiti has evolved alongside Hip Hop music, B-Boying and other elements. Gangs also use their own form of graffiti art work to mark a territory or to serve as an indicator of gang-related activities.

Here are some pictures and examples of graffiti that I have got from the Internet.