For example 34C:
34 is the Band Size which is equal the Under Bust Size (measurement of chest under the boobs) + 5 in inches.
C is the Cup Size which is equal the Bust Size (measurement of the chest through the boobs) - the Band Size.
The Cup Size is then expressed in letters AA, A, B, C, D, DD which are respectively 0 though 5 inches.
I never knew how these fancy letters and numbers were defined. Thanks sexxit!