Spend a couple hours in our Wheels/Tires sub-forum and it's stickies as well as reading TireRack's Tech section
http://www.tirerack.com/tires/...h.jsp
Here's a quick break downTire: 245/45/18 = 245 millimeters tread width, the side wall is 45% of the tread width, and 18" diameter opening for the wheel.
Wheel: 19x8.5 +15 = 19" diameter wheel, 8.5 " wide and the wheel mounting surface (mounts against the rotor/hub) is 15 mm from the center, as shown in the pic.
Negative offset wheels will have a very deep dish look, but few cars if any can support a negative offset wheel.