  In any spacetime, it is possible to have a family of observers following a
congruence of timelike curves such that they do not have access to part of the
spacetime. This lack of information suggests associating a (congruence
dependent) notion of entropy with the horizon that blocks the information from
these observers. While the blockage of information is absolute in classical
physics, quantum mechanics will allow tunneling across the horizon. This
process can be analysed in a simple, yet general, manner and we show that the
probability for a system with energy $E$ to tunnel across the horizon is
$P(E)\propto\exp[-(2\pi/\kappa)E)$ where $\kappa$ is the surface gravity of the
horizon. If the surface gravity changes due to the leakage of energy through
the horizon, then one can associate an entropy $S(M)$ with the horizon where
$dS = [ 2\pi / \kappa (M) ] dM$ and $M$ is the active gravitational mass of the
system. Using this result, we discuss the conditions under which, a small patch
of area $\Delta A$ of the horizon contributes an entropy $(\Delta A/4L_P^2)$,
where $L_P^2$ is the Planck area.
