“R2” is a short form for second release of Windows Platforms, it stands for second release of Windows Server 2008. It is not a completely new release, but rather adds additional features and refinements to the existing release (that is Windows Server 2008).
Microsoft introduced the R2 series to avoid introducing the new functionality in service packs (SPs). This also provides 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.
