Wikipedia: Graffiti (singular: "graffito," although the plural is more common) is the name for images or lettering scratched, scrawled, or more usually spray-painted on property that does not belong to the artist. Graffiti is often regarded by others as unsightly damage or unwanted vandalism.
Most of us already knew that. But what I didn't knew was that graffiti has such a longer history. Apparently it's been around since ancient times, being found in Greek(Ephesus) and Roman cities (Pompeii), as well as Mayan (Tikal), viking and varangian. During Renaissance, many of the great artists of the time carved or painted their names on the ruins of Nero's Domus Arena, thus giving birth to the grottesche style of decoration.
Although many people associate modern graffiti with hip-hop culture, graffiti predates it and has it's own culture and history. It seems that modern graffiti appeared around 1969 and reached it's peak in the mid 70's. Most of the places it was found at that time was on train and subway wagons. Some of the most notable names from that time are Stay High 149, Hondo 1, Phase2, Stitch 1, Joe 136, Junior 161, Cay 161, Barbara 62 and Eve 62. Taki 183, even if he is not the first graffiti artist, he is the first that attracted media attention.
In the period between late 1970's and early 1980's new forms and styles of graffiti developed. After 1985 it started to stagnate and because of several actions took by the US Government and the Transit Authority, who wanted to remove all traces of graffiti from their train wagons, it started to "move to the streets". This way modern graffiti, the one that each and everyone of us knows, is created.
But is graffiti a form of art? It certainly has been used for more than just defacing public or private property. Since ancient graffiti appeared, it's been used to send a message to others, being of political, social or personal nature. In modern time graffiti is used for showing others that the author has been there, much like marking territory. It's been also used for transmitting political messages, expressing an opinion, approval or disapproval for a new law, decree or election. Messages like "stop the war!" or "make piece!" are present in many of these pieces. Thus the author is expressing something through his creation. Isn't that art?
If you still disagree, know that artists like Crash, Daze and Lady Pink have their works exposed in the Brooklyn Museum, as part of the "contemporary art" gallery. Australian art historians ranked some of their local graffiti as visual art. Oxford University Press states in their "Australian Painting 1788-2000" states that graffiti has a key place in contemporary visual culture.
Still not convinced? I can't blame you. Seeing a beautiful white wall filled with awful writings is horrible. But if you see a boring white wall decorated with beautiful graphics and colors, showing a strong message that you agree with is awesome. And just so you don't have a narrow view on the entire subject, you can visit Wikipedia and find out more information about graffiti (this post was largely based on information found there). For a huge graffiti gallery, you can visit http://www.graffiti.org or http://www.puregraffiti.com