PPP - Point-to-Point
The point-to-point protocol extends the functionality of HDLC by providing Network layer encapsulation.
PPP is used primarily over dial-up or leased lines to IP networks and is the primary encapsulation method for ISDN lines.
PPP replaced SLIP (Serial Line Internet Protocol) which only worked with TCP/IP.
PPP can transfer IP, IPX, AppleTalk, and other network layer protocols.
PPP can be used over several different physical interfaces like HDLC including asynchronous and synchronous.