Gravitational waves (GWs) are ripples in space-time which carry energy and angular momentum at the speed of light. Predicted by Einstein's General Theory of Relativity, there has been to date only indirect evidence for their existence, through the observation of energy loss from binary pulsars (Weisberg and Taylor, 1984). Taylor and his student Hulse received the Nobel prize in 1993 for this proof of the existence of gravity waves. Numerous experiments have confirmed the underlying theory of General Relativity to a high degree of precision. Yet the direct observation of GWs is still necessary for the wave solutions of the Einstein's field equation to be fully investigated. More importantly, however, the ability to directly detect GWs will create a new kind of Astronomy.
Since antiquity, our only source of information about the stars has been through their electromagnetic radiation. Developments in technology have allowed us to expand our window on the universe from strictly visible variation to infrared, microwave, radio, ultra-violet, x-ray radiation. Gravitational wave astronomy offers an entirely new spectrum of radiation through which to explore the universe.
Because GWs are created by bulk motions of matter, and because normal matter is almost totally transparent to GW radiation, they offer the opportunity to listen in on regions and processes that are otherwise hidden from view, such as black hole births and the inner regions of a supernova core collapse. Whereas electromagnetic telescopes are our eyes on the universe, gravity wave detectors constitute "ears" which will allow us to "hear" for the first time the "sounds" produced by the universe. Information obtain will be complementary to electromagnetic observations, revealing processes which occur in the very core of cataclysmic astrophysical events and at the earliest moments of the Big Bang. New events will be recorded which are likely to ignite a revolution in astronomy comparable to that which followed the development of radio astronomy.
The most promising technology for gravitational wave (GW) detection is long baseline laser interferometry. A passing gravitational wave will alternately stretch then contract one arm of a Michelson interferometer whilst contracting then stretching the other arm. This slight variation is proportional to the size of the detector: it would be "as large" as the size of an atom if one could monitor the distance from the earth to the sun, and it will be about one billion times smaller in a detector several kilometres long. Such a small variation of distance can be detected through the phenomenon of interference.
For a general review on gravitational waves see the document by John S. Jacob, What are Gravitational Waves? (11 MB Power Point) and on their detection see the award winning review paper The Detection of Gravitational Waves by JuLi, GBlair and C.Zhao.