This article is from the FAQ, by with numerous contributions by others.
There are several different ways individual PC users can get access to
the Internet. Of all the access methods available, SLIP (Serial Line
Interface Protocol) and PPP (Point-to-Point Protocol) accounts give
you the most options, as far as applications are concerned.
SLIP has been around since the mid-1980s. It was originally designed
to allow Unix machines to connect to one another over the phone. It
essentially "tricks" your computer into thinking that its modem
connection is a dedicated network connection (the kind you would
usually need a network card for).
PPP is based on SLIP, but it is a more sophisticated protocol. It
contains additional error checking and authentication, which makes it
more reliable than SLIP. For most PC users, there really isn't much
difference between the two. Because PPP is more reliable and is
generally accepted to be the standard of the future, you should get
PPP if you're offered a choice between the two.
WinSock works great with SLIP and PPP. Most WinSock versions come with
dialer programs to do the actual connection over your modem.