In 1984, Apple Computer released the first Macintosh. In the ensuing eighteen years, the original operating system for that machine has been vastly extended, modified, hacked, and bloated. In 1987, Apple co-founder Steve Jobs started NeXT after being pushed out from Apple, creating a new type of computer from scratch, and basing its operating system on the BSD variant of Unix.
Ten years later, Jobs returned to Apple, bringing his NeXT cohorts with him, and in the ensuing five years they have reinvigorated the company and slowly, methodically replaced the original Macintosh OS with the NeXT OS, absorbing and then modifying a good chunk of the Macintosh user interface (but very little of the underlying guts) in the process.
Today, Apple announced that as of the beginning of 2003, their computers will start up only using the new operating system, Mac OS X, and not Mac OS 9.2, the last incarnation of the old Mac OS (or any previous version, for that matter). The only way to run old-style applications that haven't been optimized for the new OS is to use "Classic" mode, which runs Mac OS 9 inside Mac OS X.
In the long run, that's a good thing. I already run Mac OS X pretty much all the time, except to support some old hardware I should really replace anyway. But the old Mac OS was a nice ride in its time. Steve Wozniak, the other co-founder of Apple, said that "We won. Every computer in the world is basically a Macintosh now." Everyone using a computer with a mouse and icons today -- whether Windows, Mac, Linux, or something else -- owes that solid old piece of software a debt, for showing the way.