  After a long wait, gravitational wave astronomy has finally begun. Binary
black hole mergers are being detected by LIGO and Virgo, and theorists are
starting to receive a wealth of data to be analized. At this point we can at
long last begin to test alternative theories of gravity and different models of
compact objects. One powerful tool to do this is the perturbative analysis of
background spacetimes. The objective of this brief review is to introduce the
notion and analysis of black hole quasinormal modes, starting from the linear
perturbation theory and including a brief discussion of numerical methods and
astrophysical implications in the gravitational wave signals recently detected.
With these basic ingredients, more sophisticated analyses and applications are
possible.
