71cadc10's backspacing explanation
 

backspacing tells you the location of the wheel mounting surface from the inside edge of the wheel.

to measure it, you would take a rim w/o tire and lay it flat with backside facing up. Lay a straight edge across the back. Measure from the straight edge down to the surface that touches the rotor or drum when bolted up. Subtract from this the distance from the edge of the rim to the "bead" or lip where the tire would mount.

So for intance on a 8" wide wheel (8" width is measured from bead to bead), a 4" backspace means the mounting flange is exactly centered.

8" wheel w/ 3" backspace would have a deeper dish look to it from the outside.

8" wheel w/ 6" backspace would have a very shallow look to it (like most front drive cars).

much often used and confused is "offset" also. Offset is simply the difference from the mounting surface to the center of the wheel width.
so using the above examples, a 8" wheel w/ 4" backspace has zero offset

8" wide wheel w/ 3" backspace has 1" negative offset

8" wheel w/ 6" backspace has 2" positive offset

Here is a good visual explanation of wheel measurements:
http://www.rsracing.com/tech-wheel.html