What is the difference between a 700x28c and a 700x32c and a 700x..c and so on?
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.