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 & 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.
Addressing why you were able to swap 700x28s for 700x32s:
As Mike said in his answer 700x is a obsolete French system. All you really need to know is that 700c means a rim bead seat diameter of 622mm (sometimes marked as 'ISO 622').
There was also a 650x size scheme, 650b is used on road and mountain bikes and means 584mm (also known as 27.5" in the MTB world).
The other number is the approximate width of the tire when mounted on a rim and inflated. A rim of any given bead seat width will accept quite a wide range of tire widths, which is why you could fit 32mm wide tires in place of 28mm wide tires.