Virtual Private Networking is dial-up networking with a twist. Instead of dialing the number of your distant network's modem, you connect to the Internet (by modem or any other means), and then use the Internet to link you to the remote network.
This arrangement is not only cheaper (long-distance connections to the home office in Abu Dhabi can really add up), but also more reliable: When you connect via regular phone connection, many things can go wrong. But when one computer on the Internet goes down, your data is instantly and automatically rerouted to its destination.
In short, VPNs lets you make connections quickly and cheaply almost anywhere in the world. You don't need dedicated, end-to-end private lines, and you can configure extremely tight security.