Well, as much as I hate to admit that that Text 542542 commercial was right: the difference is roughly the difference between grapefruits and Oranges.
36b
36c. 36 is the measurement around your chest, below the breasts, and the letter, c, is the cup size.
right - 36DD left 36C
It's one size up it goes A B C D DD DDD/E DDDD/F etc. So its really 36A to 36B to 36C etc etc.
36c
the band might be tighter on the 34 it's essentially the same cup size for most bras though
3
36c
36C
A 34D has a larger cup size but 36C has larger bands. I wear a 34D and I think that a 36C is actually too large for me as its made for a woman with a larger bodyframe and the straps slide down my arms but some women can alternate between the two. It depends mostly on your genetics
36C
36C