  We review theory of information thermodynamics which incorporates effects of
measurement and feedback into nonequilibrium thermodynamics of a small system,
and discuss how the second law of thermodynamics should be extended for such
situations. We address the issue of the maximum work that can be extracted from
the system in the presence of a feedback controller (Maxwell's demon) and
provide a few illustrative examples. We also review a recent experiment that
realized a Maxwell's demon based on a feedback-controlled ratchet.
