SQL Server failover clustering provides high-availability support for an entire SQL Server instance. In information technology, high availability refers to a system or component that is continuously operational for a desirably long period of time. Availability can be measured relative to '100 percent operational'. High availability is achieved by a combination of hardware redundancy, fail-over systems and policies.
A SQL Server failover cluster appears on the network as a single SQL Server instance on a single computer. Internally, only one of the nodes owns the cluster resource group at a time, serving all the client requests for that failover cluster instance.