Zero Offset - The plane of the hub mounting surface is even with the centerline of the wheel.
Positive Offset - The plane of the hub mounting surface is shifted from the centerline toward the front or outside of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.
Negative Offset - The plane of the hub mounting surface is toward the back or brake side of the wheel's centerline.
"Deep dish" wheels typically have negative offset or a very low positive offset.
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.
Wheels are usually stamped with their offset using the German prefix "ET", meaning "Einpresstiefe" or, literally, "press depth". An example would be "ET45" for a 45mm offset.