The vCenter Server Appliance (vCSA) is a prepackaged and preinstalled Photon Linux-based VM that provides vCenter and PSC services. vCSA 6.5, compared to the previous version, now offers the same capabilities provided by the Windows-based version, plus some exclusive services, such as native high availability, native backup and restore, a Migration Tool, and improved appliance management.
With version 6.5, the client integration plugin (CIP) is now deprecated and no longer required, since it has been replaced by the native web client.
Since VMware has deprecated the Windows-based version of vCenter Server, use of the vCenter Server vCSA option with an embedded deployment (good enough for a small environment) or external PSC (if you have multiple vCenter servers and you want to use the Linked Mode feature across them...