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

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