  These lectures aim at providing an introduction to the properties of
gravitational waves and in particular to those gravitational waves that are
expected as a consequence of perturbations of black holes and neutron stars.
Imprinted in the gravitational radiation emitted by these objects is, in fact,
a wealth of physical information. In the case of black holes, a detailed
knowledge of the gravitational radiation emitted as a response to perturbations
will reveal us important details about their mass and spin, but also about the
fundamental properties of the event horizon. In the case of neutron stars, on
the other hand, this information can provide a detailed map of their internal
structure and tell us about the equation of state of matter at very high
density, thus filling-in a gap in energies and densities that cannot be
investigated by experiments in terrestrial laboratories.
