A cluster is a group of computers and storage devices acting as a single entity and work together and can be accessed as a single system. Clustering is a means of providing High Availability. Windows Server 2003 Enterprise Edition and Windows Server 2003 Datacenter Edition support clustering for up to eight nodes using Microsoft Cluster service. Windows Server 2003 Enterprise and Windows Server 2003 Datacenter
Operating Systems support up 8-nodes cluster.
Failover clustering with Microsoft Cluster service has many similarities and differences from Network Load Balancing. In a Windows failover cluster, two or more servers (also known as nodes) are connected to single external disk storage. Each Server has its own storage also and in most cases they are installed with "cluster aware" applications like Microsoft Exchange Server or Microsoft SQL Server. The application stores its data on the external shared storage subsystem. The application runs on only one node at a time. However, if that node experiences a hardware failure, any of the other node takes over. A person from outside can see the servers as a single server. This is referred to as active-passive clustering, where one node is active and the others are passive, waiting for the active node to fail.