What is the difference between R2 & SP?

Like me, if you have wondered how the R2 in Windows Server 2008 R2 or SQL Server 2008 R2 differs from a SP (Service Pack), the freely download-able MS Press ebook "Introducing Windows Server 2008 R2" has a good explanation - An R2 release is more than an SP, but less than a full major release.

Beginning with Windows Server 2003, Microsoft moved to a server release cycle that was designed to have a major release every three to fve years (Windows Server 2003, Windows Server 2008), with a minor release at the approximate midpoint of the major release cycle (Windows Server 2003 R2, Windows Server 2008 R2). This change allowed Microsoft to move away from including new functionality in service packs (SPs), while
providing customers with a more stable and predictable server environment. 

An R2 release is more than an SP, but less than a full major release. Windows Server 2008 R2 includes Windows Server 2008 SP2, but it also adds many new features and functionality that were not part of Windows Server 2008.

The eBook has a chapter on IIS 7.5 that may be of interest to developers, especially those getting started with Sharepont 2010.

Also see:
What do CTP, RC, RTM, RTW mean?

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