i sat there for a good 2-3 minutes re-reading your post.. then i realised it was the *other* sam... i was wondering why you were aksing what '235' meant, when you were telling us before..
the '235' etc, is the tyre width.
normally a tyre will be given a code (im not entirely sure what to call it actually), but it will looks something like this..
235/45R17
what that means, its a 235mm wide tyre, with a 45 profile rating, and it suits a 17" rim.
now the width is obvious the width of tyre that touches the road, the wider the tyre, generally the more grip you will have, but it will cost more, and you need wider rims to get really fat tyres onto them.
the 17" part is pretty easy aswell.. 17" rims uses 17" tyres.. got that?
the profile part is a bit of a prick.. what the '45 profile' means the sidewall is 45% of the width of the tyre..
so the sidewall on this particular tyre is 235 * 0.45 == 105.75mm high.
this by itself typically doesnt mean alot.. but it helps when you run different width tyres on the car (ie, you may have fatter on the rear.. like myself, and many others do)
say i want to use a 255 on the rear.. i need to find a 255 tyre that will have the same sidewall height as a 235 tyre, so to do that, i get 105.75 / 255 == 0.414, so approx '40 profile' is needed, meaning you need to use a 255/40R17 to obtain the same rolling diamter as the 235...
now that you are probably well and truely confused, i think my job is done.