The word graffiti often functions in English like a noncount—or mass—noun, which means that, like the words sand and water, it's used with a singular verb and is preceded by some rather than a or an. But graffiti is traditionally in fact a plural noun, and it has a singular related form: graffito. That's right: if you want to get all technical about it, that tag you're admiring on the abandoned building over there is a graffito.
The English word was borrowed directly from Italian in the mid-19th century. In its origin language, graffito means "incised inscription," and comes from graffiare, meaning "to scratch." It may ultimately have its origin in Latin graphium, referring to a stylus—that is, an instrument for writing, marking, or incising.