Virtual Private Networks (VPNs) aim to provide isolation between entities using the same shared infrastructure. Private networks can be “simple” GRE/IPSEC or other tunnels over the Internet, or they can be more involved Service Provider / Network manager provided approaches.
In the words of the IETF:
The term “Virtual Private Network” (VPN) refers to a set of communicating sites, where (a) communication between sites outside the set and sites inside the set is restricted, but (b) communication between sites in the VPN takes place over a network infrastructure that is also used by sites which are not in the VPN. The fact that the network infrastructure is shared by multiple VPNs (and possibly also by non-VPN traffic) is what distinguishes a VPN from a private network. We will refer to this shared network infrastructure as the “VPN Backbone”.IETF RFC 4110 A Framework for Layer 3 Provider-Provisioned Virtual Private Networks (PPVPNs)
The two main types of VPNs the IETF has concerned itself with, are: