A cluster is a purpose-built group of servers functioning modularly toward a common goal. In many situations, site or application hosting can be reasonably handled by a single server. This one server will house and maintain many services running in tandem. Services like a web service, a DNS service, a database service, and a backup service can all be responsibly handled on a single machine. Many services can share the resources of one device. As an application or site grows beyond the capacity of a single server, it’s often necessary to isolate some of those services to their own server. In a case like this, specific servers are dedicated to database processes or web server processes. At this point, when many servers are functioning toward the same goal, the group is called a cluster.