You don't need to know every graffiti term to be effective, and indeed if you overuse their language, you'll just end up looking silly. A graffiti sleuth should know the handful of words below. See here for an exhaustive list: http://en.wikipedia.org/wiki/Glossary_of_graffiti
Bomb: To create many tags in an area. Let's go on a bombing run.
Buff: The act of removing graffiti. My piece got buffed after one day!
Burn: Graffiti that has prominence and/or longevity. My piece on the water tower burned all year!
Bite: To copy someone else's graffiti. I would never bite someone's style!
Chill spots: Out of the way graffiti locations unlikely to attract the attention of the police.
Heaven spots: High and/or difficult to reach locations for graffiti.
Piece: Short for masterpiece. Elaborate graffiti. Ever tagger aspires to be a "piecer."
Piecebook: A graffiti vandal's sketchbook. The term piece refers to masterpiece. They are often carried in their backpacks. There will be stacks of these in their homes. Pro tip: these are compelling evidence.
Rack: To steal paint.
Tour: To travel to another city or country to write graffiti. Next summer I plan on going on tour in San Francisco.
Toy: A term of derision for a young inexperienced tagger.
Writer: A term graffiti vandals preferred to be called.