what is server clustering

A server cluster is a group of independent servers running Windows Server 2003, Enterprise Edition, or Windows Server 2003, Datacenter Edition, and working together as a single system to provide high availability of services for clients. When a failure occurs on one computer in a cluster, resources are redirected and the workload is redistributed to another computer in the cluster. You can use server clusters to ensure that users have constant access to important server-based resources.

Server clusters are designed for applications that have long-running in-memory state or frequently updated data. Typical uses for server clusters include file servers, print servers, database servers, and messaging servers.

(Source: Microsoft Technet)