Hello Carl,
PPTP = Point To Point Tunneling protocol is a L3 protocol that can be used to create a point to point tunnel over IP to carry PPP frames.
It has been developed by microsoft.
L2TP version 2 was made using MS PPTP and Cisco L2F.
You can find PPTP used between Windows workstations over the internet.
PPTP is defined in RFC2637
http://update.roxen.com/developers/idocs/rfc/rfc2637.html
Hope to help
Giuseppe