  A general formalism for understanding the thermodynamics of horizons in
spherically symmetric spacetimes is developed. The formalism reproduces known
results in the case of black hole spacetimes. But its power lies in being able
to handle more general situations like: (i) spacetimes which are not
asymptotically flat (like the de Sitter spacetime) and (ii) spacetimes with
multiple horizons having different temperatures (like the Schwarzschild-de
Sitter spacetime) and provide a consistent interpretation for temperature,
entropy and energy. I show that it is possible to write Einstein's equations
for a spherically symmetric spacetime in the form $TdS-dE=PdV$ near {\it any}
horizon of radius $a$ with $S=(1/4)(4\pi a^2), |E| = (a/2)$ and the temperature
$T$ determined from the surface gravity at the horizon. The pressure $P$ is
provided by the source of the Einstein's equations and $dV$ is the change in
the volume when the horizon is displaced infinitesimally. The same results can
be obtained by evaluating the quantum mechanical partition function {\it
without using Einstein's equations or WKB approximation for the action}. Both
the classical and quantum analysis provide a simple and consistent
interpretation of entropy and energy for de Sitter spacetime as well as for
$(1+2)$ dimensional gravity. For the Rindler spacetime the entropy per unit
transverse area turns out to be $(1/4)$ while the energy is zero. The approach
also shows that the de Sitter horizon -- like the Schwarzschild horizon -- is
effectively one dimensional as far as the flow of information is concerned,
while the Schwarzschild-de Sitter, Reissner-Nordstrom horizons are not. The
implications for spacetimes with multiple horizons are discussed.
