Graffiti artist Banksy is in New York for amonth long artist residency, and stencil murals have appeared in both Manhattan and Brooklyn.
The first New York mural by Banksy appeared on Tuesday, and depicts two boys holding a spray can.  It was alread painted over by the next day.
Banksy has also been doing stencil murals in Brooklyn, adding the tagline "the musical" to already existing graffiti.  Stencils spotted include "Playground Mob: The Musical" And "Occupy: The Musical".  Stencils have appeared in the neighborhoods of Williamsburg and Bushwick, an area known for its art scene and vibrant street art.
There will be a whole month of Banksy stencils for New York and Brooklyn to see, and fans can follow him around the city at the website banksyny.com

There has been much speculation about identity of Banksy, but the question remains unsettled.
Banksy directed a 2010 documentary called "Exit Through The Gift Shop" which dealt with the topic of Street Art.
Some of Banksy's most iconic works include "One Nation Under CCTV" and a depiction of a girl hugging a bomb.  Banksy's works have political and satirical dimensions.  According to Wikipedia, "Banksy once characterised graffiti as a form of underclass "revenge", or guerilla warfare that allows an individual to snatch away power, territory and glory from a bigger and better equipped enemy.  Banksy sees a social class component to this struggle, remarking "If you don't own a train company then you go and paint on one instead.""

People have argued over the topic of street art since Graffiti became a major mode of artistic expression for the hip hop community in New York City during the 70s.  Many decried tagging as vandalism, but some works of graffiti came to be recognized as having artistic merit.  
Watch Banksy in Exit Through The Gift Shop Trailer Here: