In broad terms, a cluster is a group of independent systems working together as a single system. A client interacts with a cluster as though it is a single server. Cluster configurations are used to address both availability and scalability.
Availability When a system in the cluster fails, the cluster software responds by dispersing the work from the failed system to the remaining systems in the cluster.
Scalability When the overall load exceeds the capabilities of the systems in the cluster, additional systems may be added to the cluster. At the present time, customers who plan to expand their system's capacity must make up-front commitments to expensive, high-end servers that provide space for additional CPUs, drives, and memory. Using clustering technology, customers will be able to incrementally add smaller, standard systems as needed to meet overall processing power requirements.