Citrix Provisioning Services (PVS) allows computers to be provisioned and re-provisioned in real time from a single shared-disk image. In doing so, administrators can completely eliminate the need to manage and patch individual systems. Instead, all image management is done on the master image. The local hard disk drive of each system may be used for runtime data caching or, in some scenarios, removed from the system entirely, which reduces power usage, system failure rates, and security risks.
Using provisioning services, administrators prepare a device (a master target device) for imaging by installing the required software on that device. A vDisk image is then created from the master target device's hard drive and saved to the network (on a provisioning server or storage device).
Provisioning server was going to be end of life and replaced by Machine Creation Services. Having said that, provisioning server is still very popular.