This article is from the Mac communications FAQ, by Bruce L Grubb BruceG6069@aol.com with numerous contributions by others.
Basicly Open Transport is Apple's complete revision to the Macintosh's
network system software. Originally the Mac's only native protocol
was AppleTalk and anything else had to be added on. In addition thanks
to the AppleTalk Manager (which resided in the ROM of most 68K Macs)
AppleTalk got privileged access. As a result network software developers
not only had to write each and every non-AppleTalk protocol they wanted
to use but they had to contend with AppleTalk. After Open Tranport came
out this method became known as Classic networking (not to be confused
with the Classic enviroment In MacOS X).
Open Transport which has been part of the OS since 7.5.3 changed this
situation by using industry standard Application Programing Interfaces
(APIs). Not only did the APIs eliminate the need for developers to
reinvent the wheel but they put all the protocols on an equal standing.
In addition Open Transport is Power Mac native resulting in speed ups
in both AppleTalk and TCP/IP.
Open Transport has been the the default networking system since
System 7.6 with support for Classic Networking formally disappearing
with System 8.0.
Classic and Open Transport networking are different enough that
programs written for just one generally will not work with the other.
Futhermore given the age of Classic Networking it is iffy that
programs only written for it will work under MacOS X. So read
the documentation of any networking software you plan to use to make
sure it is compatable with your networking method and OS. More details
on Open Transport can be found at Apple's web site