What they said. The front and rear axles are different widths, so the wheels have different offsets. The distance from the mounting surface to the front of the wheel is about 1" different between the fronts and rears. If you have stock wheels, or aftermarket wheels with SyTy offsets, you CAN'T put the rear wheels on the front and vice versa (without it looking funny). You have to take the tires off the wheels, and rotate the tires while leaving the wheels where they are.
If you have aftermarket wheels that are all the same, with spacers in the rear (or just left the rear wheels tucked in an inch), you can take the wheels off and rotate the whole thing.
Like TYPHOOL7 said, you're supposed to move the backs to the fronts, and the fronts to the opposite backs. If you have directional tires, you won't be able to do this without taking the tires off the wheels anyway. You could only swap the front and rear on each side.