the 1st number is how wide in millimeters the tire is acrossfrom shoulder to shoulder. so on a 215 the cross section width is 215 millimeters wide. the 2nd number is the % of height the sidewall is in comparison to the width of the tire. so a 215-45 would mean that the sidewall height is 45% of the width of the tire.
that said now you can realize that not all 35, 40, or 45 series tires are the same height. obviously like on some super cars out there that will wear something like a 315-30-18 tire on the rear, even though it is a 30 series tire it is going to have a taller sidewall height than say my 215-35...since 30% of 315 is more than 35% of 215.