A, B, C, D is the cup size, meaning that if you wear a D, you have larger breasts. The numbers 32, 34, 36, 42, stands for the band side, that connects the cups. The bands go from your sides; to your back. So, the person who wears a 38 for their band side... is larger around that area. The person who wears a D cup has larger breasts.
If you are asking about bra size, the number 36 or 38 is for how many inches around the band size is. The letter - C or D - is for the cup size. So while someone who wears a 38 is larger around the back and sides, the person who wears a D cup has a larger breast.