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.