Its PROCESSING, according to the site I got it from anyhow.
Although each of these factors plays a difference in tea type, the main variable is the type of processing technique used. White tea is minimally processed. Green tea is steamed or fired to prevent oxidation. Black tea is heavily oxidized. Oolong tea is somewhat oxidized. Puer (or pu-erh) is fermented.
Tea Types - What Differentiates Black, Green, Oolong & Other Tea Types