One of the new features of vSphere is Changed Block Tracking or CBT. With changed block tracking enabled on a virtual machine virtual disk file, the blocks changed since the last back-up are registered in a special file. This file can be read by back-up applications which are CBT-aware, like Veeam Back-Up & Replication 4 and VMware Data Recovery.
The benefit of CBT is a much faster back-up (10x to 20x faster) and replication of VM data.
Eric Siebert wrote an interesting article on CBT here.