Not going to mention names here, but someone asked me to clarify few things to my best abilities:
Sure thing:
Nas - Network Attached Storage is basically one or more hard drives in typically small case which instead of sitting inside your desktop or laptop is actually outside on your desk,under it or inside closet
It connects to network and it's main function is to serve files. Cheaper (about $300-350 w/o hard drives) units have slow and cheap controller and their functions list is limited. Higher units ($600+) have faster processors - typically Atoms, more functions and faster performance etc.
Few good articles about nas:
How To Buy a NAS - The Short(er) Version
How To Choose the Right NAS for You
NAS Basics
Windows Home Server like this
acer Aspire Easystore H340 is basically a NAS box, with one crucial exception, unless regular nas boxes which running customized linux based operating system, WHS runs actual Windows, basically dressed a up Windows 2008 Server. While it's still running relatively weak Atom process it allows to install ANY windows application or service, giving you more flexibility and ease of maintenance.
D-Stuff? I Guess you mean Boxee Box by D-Link?
Boxee Box is media client - which means is device which typical doesn't have any media on it, but gets it from network (like your pc, NAS or WHS)
But instead it capable of playing all sorts of video and audio files, support of different internet/streaming based media services like Youtube/Netflix/Pandora etc....
About HTPC - Its typically All of above in one box - Storage, Server and Media Client... Means bigger box, faster processor