PPP (Point-to-Point Protocol) and PPTP (Point-to-Point Tunnelling Protocol)
Point-to-Point Tunnelling Protocol (PPTP) is a protocol or technology that supports the use of VPN’s. Using PPTP, remote users can access their corporate networks securely, using the Microsoft Windows Platforms and other Point-to-Point Protocols (PPP) enabled systems. This is achieved with remote users dialling into their local internet security providers, to connect securely to their networks via the internet. PPP is used by PPTP to provide encryption and authentication on data packets. The main use of PPTP is to provide a tunnel for PPP, as PPP is none routable over the internet.
PPTP is a tunnelling protocol that was developed by various vendor companies including Microsoft and AS Robotics. PPTP has its issues and is considered as a weak security protocol according to many experts, although Microsoft continues to improve the use of PPTP, and claims issues within PPTP have now been corrected. PPTP is not as secure as IPsec and cannot secure two networks like IPsec can. PPTP is often replaced by Layer 2 Tunnelling Protocol (L2TP) which is another VPN protocol that provides security using IPsec, and PPTP has also been made obsolete by L2TP and IPsec. Another limitation PPTP has compared to L2TP is that it cannot route over other networks other than IP.
Although PPTP is easier to use and configure than L2TP or the IPsec framework for VPN usage, L2TP and IPsec outweigh PPTP in other areas such as being more secure and provide better stability.
Also see IPsec
Further Reading
Wikipedia's guide to PPTP (Point to Point Tunnelling Protocol)