Gravity waves are fluctuations in the gravitational field caused by the movement of mass. None have been detected yet, but it is hoped that the waves from cataclysmic events, such as neutron stars colliding, will allow astronomers to look at the universe a completely new way, hopefully creating a revoloution in astronomy as significant as the invention of radio telescopes.
Gravitational waves cannot be detected directly, as fluctuations in the weight of an object, as the fluctuations are cancelled out by the fact that there is no relative increase in weight as all objects are affected by the waves. This means that gravity waves can only be detected as tidal forces, compressing or stretching an object, perpendicular to the direction of the wave. The tidal forces produced are extremely small, with even the the largest of events causing a change in the order of one part in 1018, and most of the waves are at a magnitude of 1 part in 1022 (these are still caused by very large events). In most objects this is much smaller than the thermal motion of molecules, and in some causes, quantum effects.
Two main ideas have been proposed to detect the minute changes in shape which indicate gravity waves:
A Weber bar is a large cylinder of metal which is aligned so that its axis is pointed at a prospective gravity wave source. Normally the change in the width of the bar caused by gravitational waves is not nearly large enough to be detected, howevever, the bar is shaped so that gravity waves of a specific wavelength will cause the bar to resonate, so that the vibrations can be detected by piezoelectric sensors on the side of the bar. Modern versions of this are cooled with liquid helium to minimise thermal motion, and can detect changes to 1018, hopefully enough to detect violent events in our own galaxy.
Laser interferometery uses a very large laser interferometer to detect changes in the distances of two mirrors at right angles from one point. They have the advantage that they respond to all frequencies of gravity waves, and can be made as sensitive as desired by moving the mirrors further apart. Unfortunately, an interferometer sensitive enough to detect the changes created by gravity waves would need to be huge, and probably built in space. A smaller version of this is the LIGO (laser interferometer gravity-wave observatory) project at Caltech | 
