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 virtual networks 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.
VLANs (Virtual LANs) are logical LAN's (Local Area Networks), based on physical LAN's. A VLAN can be created by partitioning a physical LAN into multiple logical LAN's (subnets) 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 VPN (Virtual Private Network) 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.
A common example of a virtual network that is based on virtual devices is the network inside a VMware ESX virtual host that is implemented using virtual switches (vSwitches). Such networks can use non-virtual protocols such as Ethernet as well as virtualization protocols such as the VLAN protocol IEEE 802.1Q.