When to use holes instead of side terminals to wire an outlet
Shirlock has a great answer; I just wanted to point out something. There are two major different designs for back-wiring, "push-in" and "side-clamp".

Push-in wiring, sometimes branded "QuickWire", uses holes in the plastic casing, underneath which are sharp spring-loaded cleats that catch and hold the wire when you put it in. To remove them, you take a jeweler's screwdriver or a small pick or probe and push into a square tab nearby, which pushes the cleat away allowing you to withdraw the wire. Personally, I hate these things with a passion and prefer to side-wire. They only work with one gauge of wire (either 12 or 14AWG depending on the type of switch or outlet; do YOU know which one was used in every single J-box in your home?), and getting the wire back out without destroying the wire or the switch is a crap shoot. The push-release mechanism is as likely to crumple and lock the wire in as to release it, IME.

Side-clamp wiring, sometimes branded "EZ-Wire", uses a plate between the side-wiring screw and the switch body. You loosen the screw, separate the plate from the switch body (easiest to let gravity do the work; turn it plate-down and shake), insert the wire stripped to the proper length, then tighten down the side screw. It works a lot like side-wiring, except you don't have to curl the wire around the screw. They're pretty easy to use and are VERY secure, and so I like these a lot better than the push-in wiring setups. The only possible problem is, with a lot of exposed metal on the side of the switch body, you have to be certain you don't short out the switch to the bare metal ground wire, or the next switch or outlet over (very easy with the wider 20A switches and GFCI outlets). I did that a few times while repairing the electrical in my new home; the switch or plug worked fine hanging loose, but when I closed it up it stopped working. The fail-safe is to give the switch body a wrap of electrical tape, giving it an insulating layer between the switch and anything else in the box (and the box itself; most are plastic but I found a couple conductive switch/outlet J-boxes in my home).