Clustering is the use of multiple computers and redundant interconnections to form what appears to be a single, highly available system. A cluster provides protection against downtime for important applications or services that need to be always available by distributing the workload among several computers in such a way that, in the event of a failure in one system, the service will be available on another.
The basic concept of a cluster is easy to understand; a cluster is two or more systems working in concert to achieve a common goal. Under Windows, two main types of clustering exist: scale-out/availability clusters known as Network Load Balancing (NLB) clusters, and strictly availability-based clusters known as failover clusters. Microsoft also has a variation of Windows called Windows Compute Cluster Server.
When a computer unexpectedly falls or is intentionally taken down, clustering ensures that the processes and services being run switch to another machine, or "failover," in the cluster. This happens without interruption or the need for immediate admin intervention providing a high availability solution, which means that critical data is available at all times.