While people have been scrawling words on walls and bathroom stalls for ages, graffiti (singular "graffito") in their modern form were born in the 1960s around Philadelphia, but rose to prominence only during the late 1970s in New York City in connection with the birth of the Hip-Hop culture. In modern graffiti, every artist, or "writer", has a unique moniker, or "tag", which he paints on any available surface. Sometimes writers band up in "crews". Other important lingo includes "bomb", to tag as many places as possible; "piece", a large, elaborate painting of a tag or a crew's name, short for masterpiece; "buff", to remove graffiti, and "slash", to paint over another writer's tag (and no, nothing like that).
Alongside tags, other forms of graffiti exist, including political ones ranging from swastikas to the anarchy symbol, and ethnic slurs. Due to the art form's background, the latter types aren't really welcomed by writers.
Since the 1990's, street art has developed where instead of writing a tag, an artist will actually paint something recognizable (people, creatures, monsters, etc.), or paste a work on paper. Banksy and Shepard Fairey , among others, have popularized these methods.