The letters - like A, B, C, D - stand for cup sizes. The numbers - like 32, 34, 36, 38 they tell you the band size – that's the part of the bra that goes around your torso. For instance, a 34A means that your boobs are an A cup and that your bra is 34 inches around.
Example: 37 inches (bust) – 34 inches (band) = 3 inches. That’s a 34C.