P2PVPN creates a virtual private network for you which behaves like a physical network (e.g. your network at home). But instead of using an Ethernet cable to connect computers P2PVPN uses secure Internet connections. Things you normally do with your LAN (share files, play games etc.) you can also do with P2PVPN with people around the world.
OpenVPN uses a client-server architecture. If you want to set up an OpenVPN network you need a server. P2PVPN uses a peer-to-peer architecture so you do not need to setup a server to create a VPN.
P2PVPN is more like Hamachi or Wippien. However, P2PVPN is the only VPN software I know which uses peer-to-peer, has an GUI and is completely open source.