Layer 2 Tunneling Protocol, RFC2661. L2TP-over-IPsec is used on Windows and Mac OS X to resemble a regular Dial-Up connection (similar to PPTP).
Jacco de Leeuw has written a very detailed step by step guide on setting this up, located here.
Nate Carlson has made a similar L2TP/IPsec HOWTO for more advanced users.
Common Errors:
L2TP uses transport mode. As such, it has problems with it comes to NAT Traversal,
