In the early 2000's, Mac OS X had few fans, although many users harbored a love-hate relationship. Mac OS X was a direct descendant of Steve Jobs' NeXTSTEP OS, but it also was a platform and a bridge, connecting the aging and ailing Mac OS 9 into the more Internet-focused OS X world with "Classic Mode." The first three iterations of OS X were bumpy. Cheetah, Puma and Jaguar (10.0, 10.1 and 10.2, respectively) were colorful, candy-coated, transparent, pinstriped-looking operating systems, often the butt of jokes and frequent crashes. But as Apple refined the iMac line, so, too, was OS X refined, eventually giving birth to Panther, Mac OS X 10.3. Arguably, this was the first real version of OS X that offered a glimpse into the promise of the once-fledgling operating system. Panther introduced the Safari web browser and solidified the brushed-metal theme with Finder. It also represented a departure from the boring world of Windows and it introduced users to classy multiwindow management via Exposé.
The version of OS X to arrive after Panther was Mac OS X Tiger (10.4), which offered many refinements we still use today, including Spotlight, Dashboard and a host of Finder-based interface tweaks. Tiger turned the once-bubbly looking operating system into a more professional desktop environment (gone were obnoxious pinstripes and transparencies) and catered to Apple's fast-growing base of "pro-sumer" users with Quicktime 7, Core Audio and Core Image. Tiger had the fit and finish of a mature OS and it showed.
Jobs, actively anticipating a jump to Intel, had tasked Apple to build every version of OS X for x86 behind the scenes. When Apple finally bit the bullet and switched from PowerPC to Intel CPUs, Tiger for x86 gave Mac users a huge improvement in speed, functionality and reliability, essentially extending the lifespan of the OS. It is currently the only version of Mac OS X to hit 11 different updates, and it's also the first version of Mac OS X to successfully run on non-Apple hardware (thanks to hackers). Tiger on x86 also enabled Apple to build the first Apple TV, but perhaps more importantly, it provided the backbone for iPhone OS. iOS, as we know today, is based on Apple's version of Unix -- Darwin -- which is the glue holding Apple's OS together.
After the release of the iPhone in 2007, Mac OS X Leopard (10.5) debuted shortly thereafter, bringing with it Time Machine, Spaces, Quick View and an completely professional GUI upgrade including technical makeovers. Leopard complimented the iPhone world with the cover-flow Finder mode and native BootCamp integration for the now fully Intel-Macintosh line. When Mac OS X Snow Leopard (10.6) arrived, Apple killed all PowerPC capabilities in an effort to refine the whole operating system, shrinking the install size and streamlining the OS code base.
With OS X now existing on the iPhone and the desktop, Apple had unified its OS ecosystem in a way, making it easier to draw lines between iOS products and desktop counterparts. Nowhere has that become more apparent than with Mac OS X Lion (10.7), which borrowed iOS features such as inertial scrolling, multitouch and the iOS-esque Launchpad. Apple, ever looking to refine things, has since dropped the "Mac" moniker, alluding to OS X's "iOS-ificationon," which now runs rampant inside OS X Mountain Lion (10.8).
So why the history lesson? It shows the dedication Apple has to fully developing a singular product. For all intents and purposes, OS X has the same beating heart in 2012 as it did in 2001. And for that, at the very least, we should be thankful the once-hated OS has grown up to become a now-beloved desktop and mobile operating system.