The advancement of sites and especially the Internet is the fact that nowadays, they need more resources to process data more quickly. Given that the use of any machine could not meet these requirements, it appeared that the work of distributing the areas over several machines to run simultaneously would cure this problem. In here are some, we describe the various characteristics of any cluster and its own various categories. Then we will look networks (architecture, topologies, components, . . . ). Then finally we will discuss how communications are in clusters.