The lowest frequencies will consist of extremely red-shifted signals from the very early universe, as well as the slow interactions of very massive black holes, and a weak background from binary star systems. Signal frequencies often scale inversely as the mass of the relevant systems. During the twentieth century, at each opening of a new window in the electromagnetic spectrum, the universe surprised us with unexpected phenomena. Our imagination and ability to predict is limited. The sources we predict today are probably just a fraction of what we will hear when our detectors reach sufficient sensitivity.
Gravitational waves are waves of tidal force.
They are vibrations of spacetime which propagate through space at the speed of light. They are registered as tiny vibrations of carefully isolated masses. Their detection is primarily an experimental science, consisting of the development of the necessary ultra-sensitive measurement techniques. While the gravitational waves can be considered as classical waves, the measurement systems must be treated quantum mechanically since the expected signals generally approach the limits set by the uncertainty principle. This system has proved Einstein's theory of general relativity to high precision, including the quadrupole formula which states that the total emitted gravitational wave power from any system is proportional to the square of the third time derivative of the system's quadrupole moment.