From Simple English Wikipedia
Graffiti in Rome, Italy
Graffiti from Spain protesting bull-fighting
Graffiti are marks or writing made on a wall with paint or spray paint (paint that is sprayed from a can). A single mark could be called a graffito but the word graffiti is usually used, meaning that there is more than one mark.
Graffiti can take the form of art, drawings or words. When done without a property owner's permission it is considered vandalism. Sometimes it is just a person's name or a rude word. Sometimes it is as a public political protest.
Graffiti has been found in very old cities. For example, in the Roman Empire, people wrote messages and drew caricatures on walls. It was used for good instead of vandalism. Probably the simplest graffiti is when a person cuts (or carves) the name of a lover on a tree with a knife.
Today, many graffiti are very complicated mixtures of writing and pictures. They are sometimes made by gangs and applied with spray-paint to buildings, bridges, street signs, and other areas. These are often signed with a tag (in graffiti slang, a signature), which is a shape special to an artist or group.
Graffiti can be thought of as a part of hip hop or street protest. It is often but not always illegal. Some cities have small areas where graffiti can be made. Sometimes graffiti are very beautiful. Some people pay graffiti artists to do graffiti on their buildings. In the 1980s, some graffiti artists like Keith Haring became very famous.