vSphere Fault Tolerance (FT) provides protection from a host or storage hardware failures for critical virtual machines by enabling a secondary running copy of the virtual machine, running on a separate host and stored in a different datastore. The secondary virtual machine is identical to the primary protected virtual machine, and failover is instant and transparent:

vSphere FT Fast Checkpointing keeps the primary and secondary virtual machines in sync to allow the secondary virtual machine to instantly take over, should the primary virtual machine be impacted by a host or storage failure.