VPN (Virtual Private Network) is an encrypted tunnel between a private network in your company or home, and another computer or computers on the Internet. This allows users to get secure, encrypted access as if they were physically in the network (the company).
There are two possibilities. The first is dedicated to the mobile users' Mobile User VPN. Applied when an employee is off-site, perhaps working from home, and needs to access files. The user must be identified (username and password). An appropriate protocol to the VPN server in the company and a secure tunnel is created between it and the VPN Server. The computer creates a virtual employee network adapter, and gets an IP address from the local area network (LAN) software.
The second option is used when a company has more subsidiaries and wants to have all employees connected to a single virtual LAN. For example with two networks whose IP addresses are fixed, it is possible to create a tunnel between the two networks so all data tranfser is secure.