Type 2 hair includes light to very wavy hair patterns. The waves and curls in type 2 hair form in the shape of an "S" and typically stick close to the head. Wavy hair won't bounce up, even when layered.
Type 2a hair is slightly "S" waved hair that sticks close to the head. Type 2a hair tends to be fine in density. Although it does have a natural sheen, Type 2a waves usually lack volume and definition.
In Type 2b hair, the wave usually forms throughout the hair in the shape of an "S" like Type 2a, but the hair sticks closer to the head. Type 2b waves might be slightly frizzier on the crown of the head, and tends to lose curl definition easily.
Type 2c waves are the coarsest of wavy hair patterns. They are composed of a few more actual curls, as opposed to just waves. Type 2c hair tends to be more resistant to styling and loses curl definition more easily than other textures--bantu knot-outs, roller sets, or twistouts might actually cause the hair to lose definition rather than promote it.