Traditionally, Remote Desktop Services (previously called Terminal Services) provided the ability to host multiple, simultaneous user sessions on a single server. In Windows Server 2008 R2, we are expanding Remote Desktop Services to also enable a Virtual Desktop Infrastructure (VDI). Microsoft VDI provides the ability to run multiple client operating systems (OS) on a single server hosted on Hyper-V.

Personal virtual desktops are virtual machines that are permanently assigned to users by an administrator. This configuration is saved in Active Directory Domain Services. A personal virtual desktop is typically used when a user needs a dedicated virtual machine (VM) with administrative privileges (for example, a user might want to install applications).

A virtual desktop pool is a group of identically configured virtual machines that are temporarily assigned to users by the Microsoft VDI system. Administrators can configure a VM to be a part of a pool.

Source: http://blogs.msdn.com/rds/archive/2009/08/19/microsoft-vdi-overview.aspx