A computer cluster consists of a set of connected computers that work together, and thus can be viewed and operated as a single system in many respects. A Windows cluster is such a set of computers running a Windows operating system. This feature has been available since NT 4.0.

Microsoft has three technologies for clustering: Microsoft Cluster Service (MSCS), Component Load Balancing (CLB) (part of Application Center 2000), and Network Load Balancing Services (NLB). In Windows Server 2008 and Windows Server 2008 R2 the MSCS service has been renamed to Windows Server Failover Clustering and the Component Load Balancing (CLB) feature has been deprecated.

As with clusters running other operating systems, there are different types of clusters, with two primary variants.

Failover clusters are designed for high availability services and typically have a subset of the cluster "active" at any one time, while the rest are in a hot standby state, ready to take over in the event of a failure.

Parallel clusters are designed for high performance, and typically have all the members active at all times, to achieve more computations at a time. Modern supercomputers are all clustered systems, which are used to perform calculations that would take too long to be feasible on independent systems.

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