virtual private network (VPN) is a computer network in which some of the links between nodes are carried by open connections or virtual circuits in some larger network (e.g., the Internet) instead of by physical wires.

VPN(Virtual Private Network )… these are bascially the logical networks on the physical line… you can have many VPN oveer same line..
Need of VPN arises when your company need to increase the network but don’t want to buy any more switches.. take an eg. your dept. your room is packed with employees and ur company need to add 4 more persons to ur deptt. what will they do.. the solution is to create VPN’s…you can configure the switch ports in other deptts. and create a specific VLAN of ur deptt.so that the persons can sit there and access to the required pcs.