Every once in a while an Open-Jaw is more worthwhile than getting a standard round trip ticket. Open-Jaws are most generically known as trips where you travel to/from an airport from/to two different airports. To explain it would be like traveling from Miami and return to Ft Lauderdale, or travel from Ft Lauderdale to Los Angeles and return via Long Beach. If you break both sides, like going Ft Lauderdale->Long Beach and return Los Angeles->Miami, its generally considered two one way trips and not an Open-Jaw. Many airlines offer some benefits with Open-Jaws when redeeming points. The airlines all have different versions of these benefits, and many qualify Open-Jaw's as having the "open" part be the shortest leg.
In order to best use these benefits constantly look for Open-Jaw options that are cost effective, and talk to your mileage program to find out the details of their benefits on a regular basis. I would post some of them here but many of them have changed recently and I'm not sure if they'll be changing again soon.