Virtual Private Network.
Using IPSec (Internet Protocol Layer Security), a company can communicate securely in the nonsecure public Internet. IPSec is part of VPN.
For example: Consider a company that has a large number of travelling salespeople, each possessing a company laptop computer. Suppose the salespeople need to frequently consult sensitive company information (for example, pricing and product information) that is stored on a server in the company's headquarters. Further suppose that the salespeople also need to send sensitive documents to each other. VPNs can do it all.