A cluster is group of computer machines that can individually run a software. Clusters are typically utilized to achieve high availability for a server software.

Clustering is used in many types of servers for high availability.

App Server Cluster

An app server cluster is group of machines that can run a application server that can be reliably utilized with a minimum of down-time.

Database Server Cluster

An database server cluster is group of machines that can run a database server that can be reliably utilized with a minimum of down-time.