stock wheel offset
http://www.1010tires.com/WheelOffsetCalculator.asp
The offset of a vehicle's wheel is the distance between the centerline of the wheel and the plane of the hub-mounting surface of the wheel.
To maintain handling characteristics and avoid undue loads on bushings and ball joints, the car manufacturer's original offset should be maintained when choosing new wheels unless there are overriding clearance issues
http://en.wikipedia.org/wiki/Offset_%28wheel%29
Really easy way to think of it is that negative will stick out and positive will tuck in similar too stock. Varies depending on what backspace and offset is. Staying similar to stock will allow largest tire fitment without a full lift kit.

