VPN stands for Virtual Private Network. Basically it is a means of creating a secure tunnel over an untrusted or public network over which you can pass private traffic - a 'virtual' private network between the two points. You can have a point-to-point VPN between two offices for example, or a remote access VPN where VPN tunnels are set up between remote workers and a central location to allow remote access. When the VPN connection is first established there is usually some form of authentication between the endpoints, and there are various standards for this (e.g. PAP, CHAP). Once the tunnel is established there is usually encryption of the data going across it, and again there are various standards for this (e.g. IPSec). Any particular implementation of a VPN solution will pick the 'best' way of doing the job from this alphabet soup
