One thing I see a lot in tutorials and articles is people using these two names interchangeably, which IMHO is incorrect. W2K8, aka Longhorn Server is based on Windows Vista SP1. With its NT kernel being 6.0 and latest build that I know of being 6003 -- Though I read somewhere that it is getting ESU patches. Has support for IA-64, IA-32 and AMD64 architectures. W2K8 R2, aka Windows Server 7, however, is based upon Windows 7. Iikewise the NT kernel version is 6.1 and latest build I know of is 7601 -- Gets ESU patches until 2023 -- supports only AMD64 and IA-64 architectures. 2008 R2 isn't just a small add-on update like Server 2003 R2 was, it's instead a full blown separate product, with its own product keys and extra features. So, in my opinion, their names aren't interchangeable, they are NOT the same OS. Things like these trigger my OCD as heck Edit: Luckily starting with Server 2016, Microsoft completely dropped the "R2" monicker.
I think MS reached their lowest point in naming confusion with 2011 servers Windows home server 2011, Windows Small business server 2011, Windows Multipoint server 2011 and Windows Storage Server Essentials 2008R2, are all built on the same ground. To make the things worse the internal name of WHS2011 Is "home server premium" while Storage Server Essentials, which is almost identical but with more features, is called "home server standard".