The band size is measured numerically e.g 30 , 32 , 34 and refers to the size of the rib cage (right under the breasts).
The measure is in inches, for example a 30 inches band size is 76cm.
The cup size is measured alphabetically e.g AA, A, B, C, D etc and refers to the size of the breasts in relation to the rib cage i.e how much bigger the breasts are than the rib cage.