Windows Server 2008 R2 is a server operating system released by Microsoft in July 2009. It is built on Windows NT 6.1. It is the first ONLY 64-bit operating system released by Microsoft. There is no 32-bit version available for Windows Server 2008 R2.
Windows Server 2008 R2 supports Hyper-V virtulization, IIS 7.5, built-in .Net frameworks i.e. .Net Framework 3.5 etc. any many other features. The operating system is available in seven different editions: Foundation, Standard, Enterprise, Datacenter, Web, HPC Server and Itanium.
Microsoft has announced that Server 2008 R2 will be the last version of Windows supporting the Itanium architecture.