Graffiti is the action of painting and writing on surfaces, usually outside walls and sidewalks, without the permission of the owner. The Vandals are famous for their graffiti (giving us the word vandalism). The Romans carved graffiti into both their own walls and monuments and for instance Egyptian ones. On the other hand Viking graffiti can be found in Rome.
According to some, graffiti has evolved into a (still mostly illegal) art form of its own, with a modern history, master practitioners, categories of style, etc. Others regard it as simply an unwanted nuisance. Graffiti is one of the elements of the culture surrounding rap music.
To remove graffiti, high pressure cleaning can be used; it can also be painted over.
Frescos and murals are art forms which involve leaving images and writing on wall surfaces. Like the ancient cave wall paintings in France, they are not graffiti as they are created with the explicit permission (and usually support) of the owner of the walls.