Desktop Virtualization is the concept of separating a desktop environment from the physical hardware or client that is used to access it. Often allowing an organizations to use cheaper resources as the display client because actual computing is being done on a high density server resource. This allows lower end clients to be used for longer periods of time.
Virtual Desktop Infrastructure (VDI) is a desktop service allowing hosts to use their desktop on resources such as blade or remote servers. End users can access their desktops from many different locations and allowing them to have the same experience if they are at a home, office or any other office for that matter. It administrators have a single point of standardization, security policy enforcement, and administration. Allowing them to centrally maintain images as opposed to responding at the physical computers.
Remote desktop services is a windows based component which allows a user to control a remote computer as they were working on that server. Or Citrix XenApp is another similar remote terminal for services.
DESKTOP AS A SERVICE (DAAS)
Desktop as a Service (DaaS) is a cloud offering that many Cloud providers used. As the client has needs for desktop but allow a cloud provider to take the responsibility of maintaining the underlying compute, and storage infrastructure. In many cases also providing software licensing that is needed to provide seats on a monthly basis.