  Black holes are perhaps the most strange and fascinating objects known to
exist in the universe. Our understanding of space and time is pushed to its
limits by the extreme conditions found in these objects. They can be used as
natural laboratories to test the behavior of matter in very strong
gravitational fields. Black holes seem to play a key role in the universe,
powering a wide variety of phenomena, from X-ray binaries to active galactic
nuclei. In this article we will review the basics of black hole physics.
