  The potential use of the redshifted 21 cm line from neutral hydrogen for
probing the epoch of reionization is motivating the construction of several
low-frequency interferometers. There is also much interest in the possibility
of constraining the initial conditions from inflation and the nature of the
dark matter and dark energy by probing the power-spectrum of density
perturbations in three dimensions and on smaller scales than probed by the
microwave background anisotropies. Theoretical understanding of the 21 cm
signal has been fragmented into different regimes of physical interest. In this
paper, we make the first attempt to describe the full redshift evolution of the
21 cm signal between 0<z<300.
  We include contributions to the 21 cm signal from fluctuations in the gas
density, temperature and neutral fraction, as well as the Lyman alpha flux, and
allow for a post-reionization signal from damped Ly alpha systems. Our
comprehensive analysis provides a useful foundation for optimizing the design
of future arrays whose goal is to separate the particle physics from the
astrophysics, either by probing the peculiar velocity distortion of the 21 cm
power spectrum, or by extending the 21 cm horizon to z > 25 before the first
galaxies had formed, or to z < 6 when the residual pockets of hydrogen trace
large scale structure.
