How Does SQL Failover Clustering Work?
This solution requires two SQL servers (in active/passive configuration) and SAN storage for the data.
The Active SQL Server provides all SQL services with the data being served from the SAN. Between the two SQL Servers and the SAN, constant monitoring of the state of the solution is performed (known as Quorum). Should the Passive SQL Server and SAN become aware that the Active SQL Server is unavailable, the Passive Server takes over all SQL roles and becomes Active.
And when the original Active SQL Server is back online, the solution can simply be returned to its original state.