Tire size question

So 225 40 18, and 245 40 18 whats the difference , i mean was going to get 245 40 18 up front for my new tires, and jeph said there to big which i agree with, but will 225 40 18, fit my new rim?? Man i have a fealing no one will understand this thread,
i just dont want to screw up mt purchase when i buy my tires.

thanks