We give an introduction to the area of gauge/gravity duality and its
applications. Based on string theory, gauge/gravity duality maps strongly
coupled quantum field theories to weakly coupled gravity theories and thus
provides a new framework for studying physical phenomena in strongly
coupled systems. We describe the main features of this approach and
discuss its range of applicability. Moreover, we give examples of
applications in both elementary particle/nuclear physics and condensed
matter physics. The examples presented include the quark-gluon plasma,
mesons, quantum phase transitions as well as superfluids and
superconductors.