  In this work we review at an elementary level the origin, main ideas and
present status of the so called holographic principle. This principle, even in
the absence of a precise formulation, is seen by many people as a major clue
for the understanding of the relationship betweeen quantum mechanics and
general relativity. The topics considered here include black hole
thermodynamics, the information paradox, entropy bounds, the covariant
formulation, and a short discussion on the compatibility of the holographic
principle with quantum field theory and modern string theory.
