"...Similar to running multiple virtual servers on a single host, with network virtualization it is possible to run multiple isolated virtual networks on a single host. In essence this is not a new feature, because with VLANs this has already become possible in the Windows 2008 hypervisor. But VLANs – for larger organizations – have limitations (4096 to be exact) and in these environments VLAN maintenance can be error-prone and cumbersome. Network virtualization is a great feature for hosters, but many private clouds can take advantage of this in multiple ways as well. It should be noted that when a VM uses network virtualization it cannot use VLANs and vice versa..."
For more information, check out http://www.hyper-v.nu/archives/marcve/2012/08/hyper-v-network-virtualization-in-windows-server-2012/