Microsoft Hyper-V Server 2008 is a scaled back version of Windows Server 2008 with a focus on supporting virtualization with a small footprint print on your hardware. Microsoft recommends using this version for Test and Development, basic server consolidation, branch office consolidation, and hosted desktop virtualization. There are some limitations though. This is not a full blown copy of Windows Server 2008. If you need disaster recovery, business continuity, or more than 32GB of RAM, You need to look at a full copy of Windows Server 2008.
For more information, visit this website: http://www.microsoft.com/servers/hyper-v-server/default.mspx