Clusters of galaxies are the largest relaxed structures in the Universe, and have proven to be among the most important cosmological probes. They are composed of roughly 85% dark matter, 12% hot gas, and 3% stars and galaxies. In this paper, the basic properties of clusters are reviewed. The physics of the intracluster gas is described in some detail. Gases at these temperatures (10⁷–10⁸ K) mainly emit X-rays. Many clusters of galaxies have central regions of dense, cooler intracluster gas called “cool cores.” The properties and physics of cool cores are reviewed. Clusters are formed hierarchically by mergers of smaller systems. The basic physics of cluster mergers is discussed. Finally, I describe the phenomena which occur when clusters with cool cores merge. Mergers may disrupt cool cores. Cool cores in merging clusters lead to sharp density discontinuities called “cold fronts” which have proven to be one of the most common features of high-resolution Chandra images of clusters.