I first heard about street art in Current Art last semester. It is actually incredibly interesting. The idea of an artist that wants anonymity in the sense of who he actually is, yet wants credit through the name he uses to tag his artwork. The artist doesn't profit at all from his work, he just loves what he is doing, or strongly wants to get a message across.
The work they do is not seen by rich collectors, elitist critics, and those in academia; hanging on walls in a museum, or in an exclusive gallery-- it is on display for all kinds of people, in places where we all see it--driving to work, walking down a street, on public transportation.
I love the danger/risk element involved -- this art must be made at a time when no one is around to see its creation. If caught, the artist is literally breaking laws. It makes your average artist seem so much less edgy.
The street artist does not walk around in skinny jeans, vintage sweaters, wacky colors, emo hair, and wayfarers which scream "LOOK AT ME--I AM SO EDGY AND COOL AND ARTSY." (hey, I wear these things myself, I'm just sayin...) He is truly about his work and his message, and not about the hip image he gets to have as an artist.
A lot of people might think street art is mostly low art, gang related, etc. , and some of it very well may be, but there is a lot of really cool street art that is being made right now.
Banksy, based in London, is probably the most famous street artist -- check out his work, I absolutely love it.