An elementary introduction to the Holographic Principle

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.