Normally when a spacecraft is launched, its rockets are jettisoned, fall to the Earth with parachutes, and are recovered. They then undergo extensive maintenance in order to be reused.
Elon Musk's SpaceX wants to change that. Since its inception, SpaceX has been about designing, testing, and implementing reusable rockets to make spaceflight economically feasible; similar to the way that commercial airlines reuse airplanes between flights. (Imagine how expensive air travel would be if airplanes couldn't simply be refueled after landing).
SpaceX has been testing its Falcon 9 Reusable rocket successfully in launch, hover, and landing on solid ground. Now, it wants to try landing one of its rockets on a 300ft x 100ft "autonomous spaceport drone ship" (aka floating platform) in the Atlantic Ocean.
SpaceX has already attempted three "soft" landings in the ocean with their Falcon 9 rocket. After being jettisoned, the rocket falls to the Earth and re-ignites before impact to land more gently in the water. If the rocket were to land on a solid surface, it could then be refueled and ready for launch.
The first attempt at a floating platform landing is slated for December 16, 2014. SpaceX predicts a 50% chance of success. Given SpaceX's track record so far though, it seems likely that they will eventually hit their mark, if not on their first try.