Wheel Offset and Backspace Diagram
Offset - Distance from the wheel center-line (CL) to the wheel mating surface - greater positive offset moves the tires inward (narrower track)
Backspace - Distance from the inside lip of the wheel to the wheel mating surface
The tricky part is that when a wheel is widened the offset and/or backspace change (actually it is impossible for both of them to remain the same), so upgrading to wider wheels is not as straightforward as "get the same offset and backspace and they will fit." That's why God created The Tire Rack and Discount Tire.