700C is a French standard that at one time measured the outer diameter of the inflated tire - 700mm. The C was an indicator of the width of the tire. This was confusing, since it's actually the inner diameter that rests on the rim and determines wheel size, and two 700mm tires with different widths would end up having different inner diameters and requiring different sized rims.
These days, "700C" is not really an exact measure of tire size, it's more of a marketing term for "road" or "hybrid" tires, corresponding to 622mm inner diameter, but in a wide range of actual tire widths, written like "700x28C" for a 28mm-wide tire. The now-common ISO standard expresses the tire size as an exact measurement: the 700x28C tire is labelled "28-622" so there's no guessing as to what size rim is needed.
Some of favorite 700Cs in this collection include the Bombtrack Audax 700C Urban Road Bicycle (available in 5 sizes), Rivel Orlando M 700C Black Nexus 3 City Dutch Bicycle, Lombardo Sirmione L City Bicycle, and Hollandia Transport 700C City Dutch Bicycle.