An Acronym for Point-to-Point Tunneling Protocol.
PPTP is a protocol for creating VirtualPrivateNetworks that ensures that messages transmitted from one VPN node to another are secure despite being transported over an open network such as the InterNet. With PPTP, users can dial in to their corporate network via the Internet. It was developed jointly by MicrosoftCorporation, U.S. Robotics, and several remote access vendor companies, known collectively as the PPTP Forum.
If you want to support this under Linux, get PoPToP. It starts a pppd in the correct place; you might be interested in the MPPE patches.
PPTP is a great way to get onto the MetaNet (or indeed, any local network) if you're away from it and all you have is a Windows machine.
For firewalling interests: PPTP uses GRE packets (protol 47) and a TCP connection on port 1723 for control. Most FireWall/NAT implementations don't understand the GRE connection identifier and thus will only support one PPTP connection, to a single PPTP server, when your connection is over NAT. If you want to support more connections, see PPTPConnectionTracking for details.