Talk to me about Graffiti

I get a chance to see quite a bit of it on the trainride into work each day. Some of it is pretty though provoking:

“Democracy is two wolves and a lamb voting on what to eat for lunch.”

Incorrectly attributed to Ben Franklin.

But the rest of it is colorful, not particularly vulgar, shows up overnight, and is quite the artform…assuming it’s not your building it shows up on.

So, anybody know about the culture, history, beliefs, of the artists?