Windows Server 2008 and Windows Server 2008 SP2 are the same operating system, just at a different service pack level (Windows Server 2008 started at the SP1 level because it was released quite a bit after Windows Vista and SP1 was already out). As for as the windows 2008 and windows 2008 SP2 concern it’s uses the same OS kernel of windows vista, and in windows Server 2008 R2 has the same OS kernel of Windows7, also check Difference between windows server 2012 and R2
Windows Server 2008 R2 is the server release of Windows 7, so it’s version 6.1 of the OS. It introduces quite a lot of new features, because it’s actually a new release of the system.
Windows Server 2008 is based on the 6.0 kernel, the same of Windows Vista, Windows Server 2008 R2 is based on the 6.1 one, the same of Windows 7, while versioning it as 6.1 to indicate its similar build to Vista and increase compatibility with applications that only check major version numbers, similar to Windows 2000 and Windows XP both having 5.x version numbers
There are also differences at the GUI level, because Windows Server 2008 R2 uses the same new GUI introduced with Windows 7
The single most important point: Windows Server 2008 R2 exists only for 64-bit platforms, there’s no x86 version anymore.
Windows Server 2008 R2 has many features that are designed specifically to work with client computers running Windows 7. Windows 7 is the next version of the Windows operating system from Microsoft
Some of the features that only available when running Windows 7 client computers with server computers running Windows Server 2008 R2, compare to windows 2008 SP2 and windows 2008 R2, windows 2008 R2 have more Features, I will cover this in my upcoming article, also check Difference between windows server 2003 and windows 2008
Active Directory 2008 features