This paper is a review on the observational Hubble parameter data that have
gained increasing attention in recent years for their illuminating power on the
dark side of the universe --- the dark matter, dark energy, and the dark age.
Currently, there are two major methods of independent observational H(z)
measurement, which we summarize as the "differential age method" and the
"radial BAO size method". Starting with fundamental cosmological notions such
as the spacetime coordinates in an expanding universe, we present the basic
principles behind the two methods. We further review the two methods in greater
detail, including the source of errors. We show how the observational H(z) data
presents itself as a useful tool in the study of cosmological models and
parameter constraint, and we also discuss several issues associated with their
applications. Finally, we point the reader to a future prospect of upcoming
observation programs that will lead to some major improvements in the quality
of observational H(z) data.Comment: 20 pages, 6 figures, and 1 table, uses REVTeX 4.1. Review article,
accepted by Advances in Astronom
