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.