Here's an article that gives you a good general overview of virtual networking.

http://en.wikipedia.org/wiki/Virtual_network

"A virtual network is a computer network that consists, at least in part, of virtual network links.

A virtual network link is a link that does not consist of a physical (wired or wireless) connection between two computing devices but is implemented using methods of network virtualization.
The two most common forms of network virtualization are protocol-based virtual networks, (such as VLANs, VPNs, and VPLSs) and virtual networks that are based on virtual devices (such as the networks connecting virtual machines inside a hypervisor). In practice, both forms can be used in conjunction.

Virtual LANs (VLANs) are logical local area network s (LANs) based on physical LANs. A VLAN can be created by partitioning a physical LAN into multiple logical LANs using a VLAN ID. Alternatively, several physical LAN's can function as a single logical LAN. The partitioned network can be on a single router, or multiple VLAN's can be on multiple routers just as multiple physical LAN's would be. A VLAN can be on a VPN.

A virtual private network (VPN) consists of multiple remote end-points (typically routers, VPN gateways of software clients) joined by some sort of tunnel over another network, usually a third party network. Two such end points constitute a 'Point to Point Virtual Private Network' (or a PTP VPN). Connecting more than two end points by putting in place a mesh of tunnels creates a 'Multipoint VPN'.

A VPLS (Virtual Private LAN Service) is a specific type of Multipoint VPN. VPLS are divided into Transparent LAN Services (TLS) and Ethernet Virtual Connection Services. A TLS sends what it receives, so it provides geographic separation, but not VLAN subnetting. An EVCS adds a VLAN ID, so it provides geographic separation and VLAN subnetting."