The difference is the scale of the Virtualization:
- REMOTE DESKTOP SESSION HOST: Traditional RDP, Terminal Services server providing access to programs and full desktops
- Example: Server 2012 with Word installed on it may be used by 10 (or more) people to run Word through a Terminal/Remote Desktop session
- REMOTE DESKTOP VIRTUALIZATION HOST: New VDI (Virtual Desktop Infrastructure) server which hosts full end user Virtual Machines
- Example: Server 2012 hosting 10 full Virutal Machines that each (potentially) have completely different operating systems (like Windows 7 on one and Windows XP on another) and each VM (potentially) having completely different versions of Word.
If that is not clear enough, below is a snip from page 40 of the Windows Server 2008 R2 REMOTE DESKTOP SERVICES RESOURCE KIT with a much more thorough explaination.