While the article goes into more detail concerning not only the discovery but also the history leading up to it, Discovery has given a nice quick synopsis:

Gravitational waves, in their most basic sense, are ripples in spacetime. Theorized by Albert Einstein just over 100 years ago, these ripples carry gravitational energy away from accelerating massive objects in the cosmos. We can imagine gravitational waves as ripples across the surface of a pond; drop a pebble into the water and ripples travel across the surface away from the rock. Gravitational waves are similar; should two black holes collide (for example), "ripples" in spacetime will carry energy away from the impact site at the speed of light. There are indirect observations of the existence of gravitational waves, but detecting them directly has been an all but impossible task until now.

Or if you're the graphic/media type, PhD Comics has a great video and illustrated explanation.