VPN stands for Virtual Private Network, this refers to a network in which some of the parts are connected using the public Internet, but the data sent across the Internet is encrypted, so the entire network is virtually private. An example might be a company network where there are offices in different cities. Using the Internet the offices merge their networks into one network. The data is encrypted to insure that only the offices can see the data on the Internet link.
Does Rendezvous work over VPN?
Setup Mac OS X VPN Server for Mac & XP Clients
Multiple Simultaneous VPN Connections in OS X
Why Should a Remote User Route All Traffic Over a VPN?