  We give an introductory review of gauge/gravity duality, and associated ideas
of holography, emphasising the conceptual aspects. The opening Sections gather
the ingredients, viz. anti-de Sitter spacetime, conformal field theory and
string theory, that we need for presenting, in Section 5, the central and
original example: Maldacena's AdS/CFT correspondence. Sections 6 and 7 develop
the ideas of this example, also in applications to condensed matter systems,
QCD, and hydrodynamics. Sections 8 and 9 discuss the possible extensions of
holographic ideas to de Sitter spacetime and to black holes. Section 10
discusses the bearing of gauge/gravity duality on two philosophical topics: the
equivalence of physical theories, and the idea that spacetime, or some features
of it, are emergent.
