vSphere Desktop Edition is a vSphere Edition designed for licensing vSphere in VDI deployments. It can only be used for VDI deployment and can be leveraged with both VMware Horizon View and other third-party VDI connection brokers (e.g. Citrix XenDesktop)
It is only available in a pack size of 100 desktop VMs at a license list price of $6,500.00 ($65/powered on vm)
vSphere Desktop is licensed based on the total number of Powered On Desktop Virtual Machines.

vSphere Desktop Edition FAQ here: http://www.vmware.com/products/vsphere/vsphere-desktop.html
Licensing for the Horizon View family here: http://www.vmware.com/files/pdf/view/VMware-View-Pricing-Licensing-and-Upgrading-white-paper.pdf