A Virtual Private network (VPN) uses a public network (usually the Internet) to connect securely to a private network (such as a company's network) to communicate confidentially over the public network.
Using a VPN will let you share files and resources – including voice, video or data files -- as though you were physically connected on the same network. It's a good option for remote workers and organizations with global offices and partners to share data in a private manner.
To connect entire networks together via a VPN (commonly called site-to-site connections) it's best to make the connection at each location's main network router or gateway, which requires a router or a gateway with built-in VPN functionality. You'll find most VPN solutions also support remote access to users outside of the office; for example, when your employees use Wi-Fi hotspots at hotels, airports, or even their home. This requires that their computer or mobile device supports the same VPN method as your company's VPN solution.
Small Business Computing recently discussed how using a VPN benefits your small business network. This article, How to Connect Multiple Offices with VPN provides an excellent overview of VPN protocols, VPN routers, firmware and other important tech issues to consider when setting up a business VPN.