VPN is short for Virtual Private Network. This is a method of using an (insecure) intermediate network, like the Internet, to connect to separate networks and create a single virtual network.
There are many implementations that can be used to create a VPN. These software has a history of not being able to interoperate with each other since they all implemented it their own way. A protocol called IPsec was created to become the standard way of securely create a application-independant layer on top of IP. Openswan implements the IPsec protocol.
