  Black holes monopolize nowadays the center stage of fundamental physics. Yet,
they are poorly understood objects. Notwithstanding, from their generic
properties, one can infer important clues to what a fundamental theory, a
theory that includes gravitation and quantum mechanics, should give. Here we
review the classical properties of black holes and their associated event
horizons, as well as the quantum and thermodynamic properties, such as the
temperature, derived from the Hawking radiation, and the entropy. Then, using
the black hole properties we discuss a universal bound on the entropy for any
object, or for any given region of spacetime, and finally we present the
arguments, first given by 't Hooft, that, associating entropy with the number
of quantum degrees of freedom, i.e., the logarithm of quantum states, via
statistical physics, leads to the conclusion that the degrees of freedom of a
given region are in the area A of the region, rather than in its volume V as
naively could be thought. Surely, a fundamental theory has to take this in
consideration.
