What is the difference between a 700x28c and a 700x32c and a 700x..c and so on? I'm semi-new to customizing bits of bike and when looking to buy some slicker tyres for the road I came across this 700x..c system. I've previously had a set of 700x28c tyres which were then swapped for some 700x32 and they fit! I also read a post about being able to fit any 700x..c tyre to the same rim. Whats up with this &amp; what do the numbers mean?
In short - the 700x25C is a French nomenclature for tire size meaning tire diameter of 700mm and tire width 25mm and C is the width of the rim (A is very narrow, D is wide). Thus the number between "x" and "C" is the tire width in mm.

For more information you can read a section on tire sizing on Sheldon Brown page.