PPP is an acronym for Point to Point Protocol. It is a member of the TCP/IP suite of network protocols.
PPP is an extension to TCP/IP that adds two additional sets of functionality:
it can transmit TCP/IP packets over a serial link
it has login security
TCP/IP by itself cannot be transmitted over a serial link. This makes it unsuitable for WANs (Wide Area Networks). At the time of this writing it is not feasible to extend an Ethernet network over many thousands of miles although this may soon change using 10 Gigabyte Ethernet over fibre optic. Telecommunications companies however offer serial communications links around the globe right now and have done so for many years. To make TCP/IP work over these serial links, it was necessary to create a protocol that could transmit TCP/IP packets over serial lines. The two protocols that do this are:
SLIP (Serial Line Internet Protocol)
PPP is more feature rich and has largely supplanted SLIP.
When serial links that are part of the public telephone system are used, care must be taken to ensure the authenticity of all communications. To this end PPP incorporates user name and password security. Thus, a router or server receiving a request via PPP where the origin of the request is not secure, would require authentication. This authentication is part of PPP. Because of its ability to route TCP/IP packets over serial links and its authentication capabilities, PPP is generally used by Internet Service Providers (ISPs) to allow dial-up users to connect to the Internet.