Taken from Wikipedia -- Graffiti (singular: graffito; the plural is used as a mass noun) is the name for images or lettering scratched, scrawled, painted or marked in any manner on property.
In my honest opinion, it is both. But it ultimately depends on the situation and the circumstances.
Vandalism
Think about obscene graffiti. How would it look like on a public property? Say an ATM machine or on the walls of a pre-school.
Does graffiti portray a professional image in a squeaky clean environment?
Does widespread graffiti reflect well on a country? On its people?
I'm sure you guys are pretty familiar with the Michael Fay incident in '93/'94 where he was sentenced to caning for vandalism. And more recently, the vandalism of the SMRT train by Swiss National Oliver Fricker. Personally, I feel that the former was more serious and leaning towards "vandalism" because of the major damage done. On the other hand, the latter did not really cause much damage except for trespassing the secured train depot. His work was actually more of an "art", don't you agree?
You guys may beg to differ, I dunno.
Art & Music
Graffiti is a work of art. You can't deny graffiti adds life to anything.
If you notice, graffiti is also an element in the world of music. It normally associates with Street music. Hip-Hop. Grunge. Alternative.
The Gorillaz? Remember this animated band? Graffiti actually plays a role in their image, branding, and music. You can probably check out some of their earlier music videos.
Hell, even Chris Brown named his 2009 album "Graffiti".
I guess most of us agree that the cover pretty much sucked.
Came across this new duo called "Graffiti6" all the way from the UK. Look at their in-your-face use of graffiti. Their music is brilliant, by the way. Take a listen to their latest single, "Annie You Save Me".
Graffiti -- vandalism or art?
It is art. But it can be vandalism. So the moral of the story is to use graffiti the right way, at the right place and at the right time.