  Spacetimes with horizons show a resemblance to thermodynamic systems and it
is possible to associate the notions of temperature and entropy with them.
Several aspects of this connection are reviewed in a manner appropriate for
broad readership. The approach uses two essential principles: (a) the physical
theories must be formulated for each observer entirely in terms of variables
any given observer can access and (b) consistent formulation of quantum field
theory requires analytic continuation to the complex plane. These two
principles, when used together in spacetimes with horizons, are powerful enough
to provide several results in a unified manner. Since spacetimes with horizons
have a generic behaviour under analytic continuation, standard results of
quantum field theory in curved spacetimes with horizons can be obtained
directly (Sections III to VII). The requirements (a) and (b) also put strong
constraints on the action principle describing the gravity and, in fact, one
can obtain the Einstein-Hilbert action from the thermodynamic considerations.
The latter part of the review (Sections VIII to X) investigates this deeper
connection between gravity, spacetime microstructure and thermodynamics of
horizons. This approach leads to several interesting results in the
semiclassical limit of quantum gravity, which are described.
