  Spectral features in the CMB energy spectrum contain a wealth of information
about the physical processes in the early Universe, z < 2 x 10^6. The CMB
spectral distortions are complementary to all other probes of cosmology. In
fact, most of the information contained in the CMB spectrum is inaccessible by
any other means. This review outlines the main physics behind the spectral
features in the CMB throughout the history of the Universe, concentrating on
the distortions which are inevitable and must be present at a level observable
by the next generation of proposed CMB experiments. The spectral distortions
considered here include spectral features from cosmological recombination,
resonant scattering of CMB by metals during reionization which allows us to
measure their abundances, y-type distortions during and after reionization and
\mu-type and i-type (intermediate between \mu and y) distortions created at
redshifts z > 1.5 x 10^4.
