A vSphere cluster is a group of ESXi hosts. The CPU, memory, storage, and network resources of each host are combined to form a logical set of cluster resources. A vSphere cluster is required to facilitate the use of features such as vSphere HA, vSphere DRS, and Fault Tolerance.
A single vSphere 5.x cluster can contain up to 32 hosts. For vSphere features such as vSphere HA and DRS to work correctly, the configurations must be consistent across all hosts in the cluster. The consistency of shared storage and network configurations is a necessity.
