We offer three versions of Windows Server 2008r2: Web, Standard and Enterprise.
The major differences:
Web: Has DNS and web server functionality, and can be used for hosting sites and applications.
Standard: Similar to Windows office. Can be used for Office environments.
Enterprise: Is necessary for clustering functionality.
You can read the exact features of the various versions here: http://www.microsoft.com/en-us/server-cloud/windows-server/2008-r2-editions.aspx