The XenDesktop environment combines the previously-discussed virtualization solution components. It is important to understand the communications  between the different elements of the overall solution. The Desktop Delivery Controller (DDC) monitors the number of virtual desktops that are  available for logon and starts or shuts them down as necessary. The DDC stores all configuration and statistic information in the data store. Once an end user logs on and the DDC directs them to the appropriate image, the user profile is loaded from Citrix Profile management. Applications are then enumerated and delivered from the XenApp farm. Finally, licenses for the DDC, Provisioning Server, and XenApp server are verified by the license server. End user credentials are passed to the DDC from the Web Interface, which is the primary entry point for end users requesting sessions. To allow the VDI image to properly register with the DDC, the Virtual Desktop Agent must already be installed in the image. The Citrix Online plug-in is also included as part of the base image. Notice that the XenApp VHD also includes the Citrix Online plug-in to allow for enumeration of applications that may be available in other farms or servers. 

XenDesktop Architecture
XenDesktop is a complete, end-to-end solution that provides desktop delivery, desktop virtualization, and application delivery capabilities. XenDesktop comprises several Citrix component technologies that provide the essential functionality required to effectively manage, maintain, and optimize all desktop connections.