vSphere New Feature " FT "

HA, or High Availability is to ensure that if one of your hosts dies (poof, gone, power failure, hardware failure, network failure, etc) vCenter will detect the failure and then take the VM's that used to be running on the host that failed, and power up on another host in the cluser. What used to happen prior to HA was that if your host went down, those VM's were down until you repaired the host or manually registered them on another host and powered them up. HA powers them up automatically in the event of a host failure.
FT, or fault tolerance takes the concept of HA to a new level. Setting FT on a VM causes a standby VM to be setup. That VM is updated constantly so that in the case of a host failure, the standby VM immediately assumes processing. So, from a host OS perspective there is no power failure. The transition from primary FT VM to standby FT VM is nearly instantaneous. This ensures that there is no downtime for the FT VM.