In simple terms, Graffiti is an art form that is practiced mainly for civil disobedience. While uses change over time and culture, most graffiti is placed in secret, against the wishes of a larger populace, and is intended to provoke emotion in both lawmakers and the general public. Graffiti is commonly seen on walls and streets in urban areas, but it is also popular on bridges, factories, and rural areas where it might go unnoticed by the authorities.
Graffiti is traced back to Ancient Man; cave paintings are the earliest form of proper graffiti and used primitive pigments and tools. Graffiti has been found on ancient buildings dating from the Roman Empire, as well as on monuments in Egypt and landmarks in the U.S.
Modern day graffiti is often associated with urban culture and gang symbology; however, it is more often placed by a single person attempting to gain favor (street-cred) with a peer group. Signing names and other phrases is known as "tagging" and it is considered very admirable to tag a public building in plain sight.
Graffiti is considered a public nuisance by most governments and is a drain on public funds; local businesses dislike having graffiti nearby, as it depreciates the social value of the area. However, graffiti is a largely non-violent and non-damaging method of self-expression, and as such is less of concern to public safety than other activities.
If you go back thousands of years in the past, Graffiti has been used among the primitive in countries such as Egypt, Greece, French, and etc. Graffiti was found in caves and they were pictures of animals, people, or landmarks to show future generations that these were the types of animals they hunted, or this part of the cave or area belonged to them, and simply just for documentation purposes.