  We present a precise formulation of a correspondence between information and
thermodynamics that was first observed by Szilard, and later studied by
Landauer. The correspondence identifies available free energy with relative
entropy, and provides a dictionary between information and thermodynamics. We
precisely state and prove this correspondence. The paper should be broadly
accessible since we assume no prior knowledge of information theory, developing
it axiomatically, and we assume almost no thermodynamic background.
