A cluster of computers refers to a group of interconnected computers that are working together as one unit. It is a relatively simple concept and it has been around, arguably, for a very long time. Within a cluster, individual computers can be coupled either loosely or tightly.

• A loosely-coupled cluster consists of computers that are running with a minimum of communication and cooperation among them. This results in efficient usage of each individual computer, but limits the amount of coordination and sharing of workloads.
• A tightly-coupled cluster consists of a group of computers that are cooperating to a great degree among each other, coordinating and sharing workload, and communicating status details on a continuous basis.