To make it simple, 2 is wavy and A, B, and C refer to the texture and density of the hair. 3 is curly and A, B, and C deals with the tightness of the curl. A is loose, big, shiny curls. B is medium curls (like ringlets). C is tightly curled. And type 4 is the tightest curl of all. A is an "S" pattern and B is a "Z" pattern.
Oddisy's hair falls into mostly 3B as her curls are medium in size, and when her hair was shorter it was still curly. Had her hair been 3A, her hair would have been straighter when short. But she may have a few 3A curls thrown in amongst the 3B. Most curlies have several different types on their head!
Anyway, I hope that helps. Go to the link above for greater detail.