700C today refers to a rim with a bead seat diameter (BSD) of 622mm. Many years ago there were 700A, 700B, 700C and 700D wheels. Each had a different size rim and tire set that resulted in a nominal diameter of 700mm. Modern 700C wheels are used with tires from 18mm (super narrow racing tires) to ~75mm wide (wide '29er' mtb tires which also use 700C rims) so there is great variation in the diameter. But the rim size remains the same.
(edit 2) 700A, B, and D are obsolete. GT made a few hybrid/cruiser bikes a while back with 700D wheels, and if you have one of those today and the tires wear out you basically need to buy a new bike. Other wheel sizes that used a similar system are 650B (medium width tires used on old dirt path and 'camping bikes, I think, as well as many modern mtbs) and 650C (small racing wheels used for an aerodynamic advantage in time trials and triathlons)
(edit 3) Read this:
http://sheldonbrown.com/tire-sizing.html