VPN : Java Glossary

VPN creates a virtual site-to-site encrypted link between two computers attached to the Internet, that behaves much like an IP link between two computers over a LAN. They can use that link to communicate in any Internet protocol.

VPN uses SSL/TLS as the cryptographic layer.

OpenVPN tunnels traffic over UDP port 5000. This means you must open up your firewall-router to let that traffic through. The TUN driver passes IP traffic and the TAP driver passes Ethernet traffic. You might use one or the other or both. The advantage of VPN is that it is invisible to hackers. Packets not properly encrypted are simply ignored.