Offset is the difference between the mounting face of the wheel (the part that fits against the brake drum or disc) and the centerline of the wheel. Positive offset puts the wheel farther into the wheelwell (toward the center of the car). Negative offset puts the wheel farther out toward the outer edge of the wheelwell. A wheel with 0 offset will center the tire in relation to the brake drum or disc.
Backspacing is the distance from the mounting surface of the wheel to the very back or inside edge of the wheel. Lay a bare wheel face down & put a straight edge across the back side. Measure up from the mounting face of the wheel to the straight edge. This is the backspacing.
Keep in mind that wheel width is measured from the inside edges of the rim, where the tire bead seats. A wheel is usually about an inch wider from outside edge to outside edge than it's specified size. Example, a 7" wheel will usually measure about 8" across. Confused now?