There are many specific terms associated with the various angles and measurements involved in aligning the suspension of an automobile. The drawing below should help visualize some of those.
Wheels are usually built so that the actual mounting surface (to the hub) is well outboard of the wheel centerline. This both helps clear the brake assembly and also places the center of loading closer to the wheel bearings. Thus, the wheel's offset, often measured in millimeters, is the amount the mating face has been moved outward from the centerline.
If wider tires which interfere on the inside sidewall with shock dampers or other suspension components are mounted, spacers can be added between the hub and the wheel itself. This has the effect of actually reducing the offset. That is, a wheel whose offset is marked "ET32" used with a 10 mm. spacer fits the same as a wheel marked "ET22".