Microsoft Cluster Server (MSCS) is software designed to allow servers to work together as one machine, to provide failover and increased availability of applications, or parallel calculating power in case of high-performance computing (HPC) clusters (as in supercomputing).
Windows NT Server, Enterprise Edition was the first version of Windows server to include the MSCS software. The software has since been updated with each new Enterprise Edition server release: Windows 2000 Advanced Server, Windows 2000 Datacenter Server, Windows Server 2003 Enterprise Edition, and many new features are added in Windows Server 2008 and 2008 R2. The cluster software evaluates the resources of servers in the cluster and chooses which are used based on criteria set in the administration module. In June 2006 Microsoft also released Windows Compute Cluster Server 2003, the first high-performance computing (HPC) cluster technology offering from Microsoft.
Unlike other clusters which are made for better performance, Windows clusters have historically only presented the option of failover services, which means that if a failure occurs on a server that is a member of the cluster (Cluster node) the services that the failing server was hosting will automatically restart themselves on another server that is a member of the same cluster. The process of a service moving from one server to another is called Failover.