The offsets for the OEM wheels are front: 49 and rear: 52. Offset is the distance from the mounting point of the wheel to the center of the wheel (usually measured in mm). Basically, using the stock offsets as an example, a wheel with a lower offset, lets say 40, will stick out more (9mm front, 12mm rear) than the stock wheels. Most people like to buy wheels with lower offsets than the stock wheels because they like the wheels to be flush with the fender or even poke out a little bit.
This may be helpful:
http://www.tirerack.com/wheels/tech/...jsp?techid=101