Microsoft Knowledge Base Article 308208 contains the following summary:
A virtual private network (VPN), allows you to connect components to a network, via another network, such as the Internet. You can make your Windows 2000 Server-based computer a remote-access server so that other users can connect to it by using VPN, and then access shared files on your local drives or on your network. Virtual private networks accomplish this by "tunneling" through the Internet or another public network in a manner that provides the same security and features as a private network. With a VPN, connections across the public network can transfer data using the routing infrastructure of the Internet, but to the user it appears as though the data were being sent over a dedicated private link.
This article describes how to install virtual private networking (VPN) and how to create a new VPN connection in Windows 2000.