Bra sizes consist of a number and a letter. The number (usually ranging from 32-40) measures the band size in inches. The letter measures the bra cup size. It usually ranges from A-D, with A being the smallest, and D being the biggest. The average size is usually C. Some specialty brands make sizes G-K.
For example, my size is 38DDD. That means that my bra's band is 38 inches around, and my cup size is DDD (fairly large.)