vSphere HA provides a base level of protection for your virtual machines by restarting virtual machines in the event of a host failure. vSphere Fault Tolerance provides a higher level of availability, allowing users to protect any virtual machine from a host failure with no loss of data, transactions, or connections.
Fault Tolerance provides continuous availability by ensuring that the states of the Primary and Secondary VMs are identical at any point in the instruction execution of the virtual machine. This is done using the VMware vLockstep technology on the ESXi host platform. vLockstep accomplishes this by having the Primary and Secondary VMs execute identical sequences of x86 instructions. The Primary VM captures all inputs and events (from the processor to virtual I/O devices) and replays them on the Secondary VM. The Secondary VM executes the same series of instructions as the Primary VM, while only a single virtual machine image (the Primary VM) executes the workload.
If either the host running the Primary VM or the host running the Secondary VM fails, an immediate and transparent failover occurs. The functioning ESXi host seamlessly becomes the Primary VM host without losing network connections or in-progress transactions. With transparent failover, there is no data loss and network connections are maintained. After a transparent failover occurs, a new Secondary VM is respawned and redundancy is re-established. The entire process is transparent and fully automated and occurs even if vCenter Server is unavailable.