This article is from the Mac communications FAQ, by Bruce L Grubb BruceG6069@aol.com with numerous contributions by others.
SLIP stands for Serial Line Internet Protocol. SLIP was a "non-standard"
for framing IP packets and shipping them over a serial line (e.g. a
cable, or a pair of modems), thus allowing a home machine to dial up and
become part of the Internet. Effectively, SLIP turned a serial port into
a logical Ethernet port. PPP (see below) has effectively replaced SLIP
as the standard of choice for Internet connections.
CSLIP stands for Compressed SLIP. CSLIP reduces the size of the
headers in IP packets by eliminating a certain amount of redundancy.
This improves interactive performance.
Synergy Software offers a CSLIP MacTCP extension with its
VersaTerm/VersaTerm-PRO packages. If you already own Versaterm,
SLIP is a $20 upgrade. If you buy the complete VersaTerm 5.0.4
package, you will also get an FTP server and client, a Telnet
connection tool, and MacTCP. Performance is comparable to that
of MacSLIP. Contact: [USA] (215) 779 0522
PPP stands for Point-to-Point Protocol. PPP has been stated as a
standards-track protocol by the Internet Engineering Task Force and
the Internet Activities Board. PPP can support both synchronous
and asynchronous connections and protocols that are not IP-based
(such as AppleTalk). It provides specifications for error detection,
feature negotiation, escaping control characters, etc. As a result
PPP has become the defacto standard for connecting to the Internet
with FreePPP being the most popular extension for Classic Networking.