  Clusters of galaxies are the most massive objects in the Universe and precise
knowledge of their mass structure is important to understand the history of
structure formation and constrain still unknown types of dark contents of the
Universe. X-ray spectroscopy of galaxy clusters provides rich information about
the physical state of hot intracluster gas and the underlying potential
structure. In this paper, starting from the basic description of clusters under
equilibrium conditions, we review properties of clusters revealed primarily
through X-ray observations considering their thermal and dynamical evolutions.
The future prospects of cluster studies using upcoming X-ray missions are also
mentioned.
