A Virtual Private network (also known as a VPN) allows the user to create a secure tunnel from their location to a specified location elsewhere on the internet.
This is most commonly utilised when users wish to connect to a remote site to access sensitive data as if they were on the network/PC at the far end.
TaxCalc users commonly utilise a VPN to run TaxCalc while away from the office. The remote machine initiates a VPN connection to a device on the network (most commonly the router or server) and establishes a secure end to end "tunnel". The remote user could then use the network copy of TaxCalc as any other PC on the network at the far end could (subject to bandwidth limitations).