  The linear anisotropies in the temperature of the cosmic microwave background
(CMB) radiation and its polarization provide a clean picture of fluctuations in
the universe some 370 kyr after the big bang. Simple physics connects these
fluctuations with those present in the ultra-high-energy universe, and this
makes the CMB anisotropies a powerful tool for constraining the fundamental
physics that was responsible for the generation of structure. Late-time effects
also leave their mark, making the CMB temperature and polarization useful
probes of dark energy and the astrophysics of reionization. In this review we
discuss the simple physics that processes primordial perturbations into the
linear temperature and polarization anisotropies. We also describe the role of
the CMB in constraining cosmological parameters, and review some of the
highlights of the science extracted from recent observations and the
implications of this for fundamental physics.
