PPP (the Point to Point Protocol) is a mechanism for creating and running IP (the Internet Protocol) and other network protocols over a serial link - be that a direct serial connection (using a null-modem cable), over a telnet established link, or a link made using modems and telephone lines (and of course using digital lines such as ISDN).
Using PPP, you can connect your Linux PC to a PPP server and access the resources of the network to which the server is connected (almost) as if you were directly connected to that network.
You can also set up your Linux PC as a PPP server, so that other computers can dial into your computer and access the resources on your local PC and/or network.
As PPP is a peer-to-peer system, you can also use PPP on two Linux PCs to link together two networks (or a local network to the Internet), creating a Wide Area Network (WAN).
One major difference between serial based PPP and an Ethernet connection is of course speed - a standard Ethernet connection operates at 10 Mbs (Mega - million bits per second) or 100 Mbs maximum theoretical throughput, whereas an analogue modem operates at speeds up to 56 kbps (kilo - thousand bits per second).
Also, depending on the type of PPP connection, there may be some limitations in usage of some applications and services.
PPP is strictly a peer to peer protocol; there is (technically) no difference between the machine that dials in and the machine that is dialed into. However, for clarity's sake, it is useful to think in terms of servers and clients.
When you dial into a site to establish a PPP connection, you are a client. The machine to which you connect is the server.
When you are setting up a Linux box to receive and handle dial-in PPP connections, you are setting up a PPP server.
Any Linux PC can be both a PPP server and client - even simultaneously if you have more than one serial port (and modem if necessary). As stated above, there is no real difference between clients and servers as far as PPP is concerned, once the connection is made.
This document refers to the machine that initiates the call (that dials in) as the CLIENT, whilst the machine that answers the telephone, checks the authentication of the dial in request (using user names, passwords and possibly other mechanisms) is referred to as the SERVER.
The use of PPP as a client to link one or more machines at a location into the Internet is, probably, the one in which most people are interested - that is using their Linux PC as a client.
The procedure described in this document will allow you to establish and automate your Internet connection.
This document will also give you guidance in setting up your Linux PC as a PPP server and in linking two LANs together (with full routing) using PPP (this is frequently characterized as establishing a WAN - wide area network - link).