C as in 700C is a leftover from an old french sizing system. Doesn't say much today. Sheldonbrown.com has nice writeup about tire sizing. It's better to stick with the ETRTO marking of diameter (in mm) and width (in mm). A 700C tire will be 622 x 23-38 mm. 23 mm width for a road bike, 38 mm for a hybrid.
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