"A virtual private network (VPN) is a private data network that makes use of the public telecommunication infrastructure, maintaining privacy through the use of a tunneling protocol and security procedures. A virtual private network can be contrasted with a system of owned or leased lines that can only be used by one company. The main purpose of a VPN is to give the company the same capabilities as private leased lines at much lower cost by using the shared public infrastructure.
Companies today are looking at using a private virtual network for both extranets and wide-area intranets.

The main reason that companies use secure VPN is so that they can transmit sensitive information over the Internet without needing to worry about who might see it. Everything that goes over a secure VPN is encrypted to such a level that even if someone captured a copy of the traffic, they could not read the traffic even if they used hundreds of millions of dollars worth of computers." *

* vpnc.org/VPN Technologies