In case you wonder, as I did a few days ago, what Hyper V is, here is a simplified answer (the Wikipedia will tell you more):
Hyper V is kind of like Virtual PC for Servers on Steroids: it allows you to efficiently run and manage a whole bunch of different VMs on one machine. Hyper V is a role that is added to Windows 2008 Server (or Windows 2008 Server R2). Once you add the role, some strange things actually happen to your Windows Server installation: it actually becomes it’s own ‘parent partition’ on the server, so that it is also virtualized along with the VMs that you create.
There are many differences between Hyper V and Microsoft Virtual PC. First of all, the VMs have access to way more virtual hardware: multiple processors (Virtual PC only gives you one processor, even on a many core machine), and much more memory. It also requires Hardware Virtualization Assistance, which is present on most modern manycore processors, but if often turned off in the BIOS. Hyper V also comes with all kinds of nifty management tools, and is trying to get into the datacenter-level virtualized server market.