PPP (Point to Point Protocol) is a mechanism for creating and running the Internet Protocol - IP (and other network protocols) over a serial link - be that a direct serial connection or a link made using modems and telephones. 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.
One major difference is of course speed - an Ethernet connection operates at 10 M bits per second (maximum theoretical throughput) whereas a modem operates at speed up to 33.6k bits per second. Depending on the type of PPP connection, there may be some limitations in usage of some applications and services.
When you dial into a site to establish a PPP connection, you are a client. The machine to which you connect is called the server.
This document describes the process of setting up a PPP client. When you are setting up a Linux box to receive and handle dial in PPP connections, you are setting up a PPP server, which is NOT covered in this document (but will be added later).
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). There is no real difference between clients and servers as far as PPP is concerned, once the connection is made. This means that PPP can (and is) used to link together two or more local areas networks - LANs, or to link one or more LANs into the Internet.
In the interests of clarity, 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 ids, passwords and possibly other mechanisms) is referred to as the SERVER.
Probably the use of PPP as a client that most people are interested in is to link one or more machines at a location into the Internet. The procedure described in this document will allow you to establish and automate your Internet connection.