To make the transition from Mac OS to Mac OS X easier, Apple provided developers with an API called Carbon which let them build applications that would run on both Mac OS and Mac OS X. The Classic Environment was removed in Mac OS X 10.5 released in 2007. I remember successfully running an old application from 1984 in Mac OS X 10.4. Old applications could still be run within the Classic Environment, an application that let you boot and run Mac OS 9 inside Mac OS X.
In 2001, Apple released a completely new operating system called Mac OS X (pronounced 'ten'). Older applications built for the 68k continued to work through emulation, something that was completely transparent to the user. In 1994, Apple switched to the PowerPC processor, developed by Apple, IBM and Motorola. The Mac on the other hand tends to go through big changes every 5–10 years. I'd say Microsoft are quite extreme when it comes to backwards compatibility.
