Windows Server 2008 R2 Foundation is a cost-effective, entry-level technology foundation targeted at small business owners and IT generalists supporting small businesses. Foundation is an inexpensive, easy-to-deploy, proven, and reliable technology that provides organizations with the foundation to run the most prevalent business applications as well as share information and resources.

Windows Server 2008 R2 Standard is the most robust Windows Server operating system to date. With built-in, enhanced Web and virtualization capabilities, it is designed to increase the reliability and flexibility of your server infrastructure while helping save time and reduce costs. Powerful tools give you greater control over your servers, and streamline configuration and management tasks. Plus, enhanced security features work to harden the operating system to help protect your data and network and provide a solid, highly dependable foundation for your business.

Windows Server 2008 R2 Enterprise is an advanced server platform that provides more cost-effective and reliable support for mission-critical workloads. It offers innovative features for virtualization, power savings, and manageability and helps make it easier for mobile workers to access company resources.

Windows Server 2008 R2 Datacenter delivers an enterprise-class platform for deploying business-critical applications and large-scale virtualization on small and large servers. Improve availability, enhance power management, and integrate solutions for mobile and branch workers. Reduce infrastructure costs by consolidating applications with unlimited virtualization licensing rights. Scale from two to 64 processors. Windows Server R2 2008 Datacenter provides a foundation on which to build enterprise-class virtualization and scale-up solutions.

Windows Web Server 2008 R2 is a powerful Web application and services platform. Featuring Internet Information Services (IIS) 7.5 and designed exclusively as an Internet-facing server, it offers improved administration and diagnostic tools to help reduce infrastructure costs when used with a variety of popular development platforms. With included Web Server and DNS Server roles, as well as improved reliability and scalability, this platform allows you to manage the most demanding environments — from a dedicated Web server to an entire Web server farm.

Windows HPC Server 2008, the next generation of high-performance computing (HPC), provides enterprise-class tools for a highly productive HPC environment. Windows HPC Server 2008 can efficiently scale to thousands of processing cores and includes management consoles that help you to proactively monitor and maintain system health and stability. Job scheduling interoperability and flexibility enables integration between Windows and Linux based HPC platforms, and supports batch and service oriented application (SOA) workloads.

Windows Server 2008 R2 for Itanium-Based Systems delivers an enterprise-class platform for deploying business-critical applications. Scale database, line-of-business, and custom applications to meet growing business needs. Help improve availability with failover clustering and dynamic hardware partitioning capabilities.* Virtualize deployments with rights to run an unlimited number of virtual instances of Windows Server.** Windows Server 2008 R2 for Itanium-Based Systems helps provide the foundation for a highly dynamic IT infrastructure.
*Requires supporting server hardware.
** Requires a third-party virtualization technology. Hyper-V is not available for Itanium-based systems.

http://www.microsoft.com/windowsserver2008/en/us/r2-editions-overview.aspx