vSphere Replication copies a virtual machine to another location, within or between clusters, and makes that copy available for restoration through the vSphere Web Client or through a full disaster recovery product such as
Site Recovery Manager. vSphere Replication protects the virtual machine on an ongoing basis. Changed blocks in the virtual machine disk(s) for a running virtual machine at a primary site are sent via the network to a secondary site, where they are applied to the virtual machine disks for the offline (protection) copy of the virtual machine.
It enables the use of heterogeneous storage across sites and reduce costs by provisioning lower-priced storage at failover location.
vSphere Replication is typically limited to 500 protected virtual machines.

VMware introduced vSphere Replication with vSphere 5 and SRM 5 (hypervisor-based replication product). Prior to vSphere 5.1 this was only a feature enabled with Site Recovery Manager (SRM), a fee-based extension product - not the base vSphere product. With 5.1 this feature became available with the standard vSphere editions (maintained with 5.5).

vSphere Replication in 6.0– Improved Scale and Performance for vSphere Replication. Improved Recover Point Objectives (RPOs) to 5 minutes. Support for 2000 VM replication per vCenter.

vSphere Replication in 5.5:
- New user interface: You can access vSphere Replication from the Home screen of the vSphere Client. The vSphere Replication UI provides a summarized view of all vCenter Server instances that are registered with the same SSO and the status of each vSphere Replication extension.
- Multiple points in time recovery: This feature allows the vSphere Replication administrator to configure the retention of replicas from multiple points in time. You can recover virtual machines at different points in time (PIT), such as the last known consistent state.
- Adding additional vSphere Replication servers: You can deploy multiple vSphere Replication servers to meet load balancing needs.
- Outgoing and Incoming views.
- Interoperability with Storage vMotion and Storage DRS on the primary site: You can move the disk files of a replicated source virtual machine using Storage vMotion and Storage DRS, with no impact on the ongoing replication.
- vSphere 5.5 includes VMware Virtual SAN as an experimental feature. You can use VMware Virtual SAN datastores as a target datastore when configuring replications, but it is not supported for use in a production environment.
- Configure vSphere Replication on virtual machines that reside on VMware vSphere Flash Read Cache storage. vSphere Flash Read Cache is disabled on virtual machines after recovery.

vSphere Replication overview: http://www.vmware.com/files/pdf/vsphere/VMware-vSphere-Replication-Overview.pdf
vSphere Replication Limits: http://bit.ly/16PJKTe