OS X has deep roots, to OSes that predate Steve Jobs' return to Apple. In fact, the roots of OS X date back to around the time Jobs was thrown out of Apple, and decided to start a new technology company called NeXT.
NeXT started life selling advanced computer workstations that ran an object-oriented operating system called NeXTSTEP. NeXTSTEP was based on the Mach kernel, an OS developed at Carnegie Mellon, and BSD (Berkeley Software Distribution), a UNIX derivative developed at UC Berkeley.
That places some of the key development of OS X's core technology as far back as the 1970s. But that was only the basis for NeXTSTEP and later, OS X; the real innovation was the object-oriented frameworks incorporated into the OS, and passed along to OS X.
With the basic background of OS X out of the way, let's take a look at each OS X release.
OS X Public Beta
On September 13, 2000, Apple released a public beta of OS X. This preview version sold for $29.95. I remember eagerly grabbing a copy and installing it on a Power Macintosh G3 minitower with 384 MB of RAM and an ATI 3D Rage Pro graphics card.
The Aqua user interface was brand new back then, with its opalescent set of control buttons in the left-hand corner of every window. Steve Jobs said, "We made the buttons on the screen look so good you'll want to lick them."
I don't remember ever wanting to lick them, but compared to the somewhat drab appearance of OS 9.x, the OS X beta was an incredible explosion of color and function.
OS X 10.0 Cheetah
Apple released OS X 10.0 on March 24th, 2001. While Cheetah was the development name for the OS, Apple mostly referred to it by the numbering scheme. This initial release of OS X was a bit buggy, and was known for producing kernel panics at the drop of a hat.
There was also the issue of a dearth of applications that could run natively in OS X. Luckily, the OS could jump back and forth between the Classic version of the Mac OS and the newfangled OS X version.
OS X 10.1 Puma
Released in September 2001, OS X 10.1 was a much improved version, with many bug fixes, plus features, such as the ability to play back a DVD, that the original version lacked.
OS X 10.1 was a free upgrade for 10.0 users, and $129 for new users.
OS X 10.2 Jaguar
Jaguar marked the first time that Apple officially used the cat-based code names as part of the public branding of OS X. At its introduction, Steve Jobs tended to pronounce the name as jag-u-waarr, which while not exactly wrong, wasn't the expected pronunciation. He took a good deal of ribbing in the media about that.
Released on August 23, 2002, Jaguar marked the first version of OS X with hefty performance gains as well as many features we now take for granted, including the first version of iChat (now called Messages) and Address Book (now called Contacts). It also included support for what was then pretty impressive graphics, on ATI or NVIDIA AGP-based graphics cards.
OS X 10.3 Panther
Panther was released on October 24, 2003, and once again, the OS delivered remarkable performance increases. Some of its new features were also quite remarkable, including built-in video conferencing with iChat AV and the introduction of Safari for web browsing (earlier versions of OS X used Internet Explorer for the Mac).
Panther also marked the first time that Apple started dropping support for earlier PowerPC-based versions of the Mac. My PowerMac G3 was no longer supported, but a little fiddling with the installer could get it to work.
OS X 10.4 Tiger
On April 29, 2005, Apple introduced Tiger. Among the new features introduced with Tiger were Spotlight, Smart Folders, QuickTime 7, and Automator, along with Core Image and Core Video (APIs that developers could use to create image- and video-rich applications).
During the time Tiger was being sold, Apple introduced Intel-based Macs that included a special version of Tiger (10.4.4). The retail version of Tiger could only boot PowerPC-based Macs; the version of Tiger included with Intel-based Macs could boot both Intel and PowerPC Macs.
This led to quite a bit of confusion among users who upgraded to Intel-based Macs, and later tried to install the original version of Tiger.
Apple released Leopard on October 26, 2007, marking what may well be the most comprehensive upgrade for OS X yet. The Leopard DVD could boot PowerPC Macs and Intel Macs; it also worked with 64-bit versions of the Mac processors.
Leopard introduced Time Machine, Boot Camp, for running Windows, and Spaces. However, it also bid goodbye to the Classic environment, leaving many older Mac OS applications unable to run under Leopard, and many sad gamers.
Snow Leopard, introduced on August 28, 2009, marked two turning points for Mac users. First, it was the last version of OS X to be created independent of iOS user interface elements and services. And second, it marks the first time that OS X was predominantly referred to by its name rather than its version number.
Snow Leopard received a rewrite of the Finder using native Cocoa API instead of the much older Carbon API. The switch to Cocoa provided a more responsive Finder. Snow Leopard also received improvements through the use of multiple processors and cores, with a central service (Grand Central Dispatch) taking care of CPU utilization for developers.
Snow Leopard also introduced the Mac App Store, an online store for purchasing Mac-based applications, much like the iTunes App Store for iOS devices.
Released on July 20, 2011, Lion marks the point where OS X and iOS started to converge. Apple used Lion to start bringing elements of iOS to the Mac, including Launchpad (a type of application launcher), disappearing scroll bars, natural scrolling (which seemed very unnatural to me), Resume, and Auto-Save. iCloud, which began life as a replacement for Mobile Me, also made its official appearance as part of the OS instead of a separate service.
Lion also removed support for Rosetta, which had allowed OS X apps written before Intel Macs came along to continue to operate. The loss of Rosetta meant that many games and even some major productivity apps stopped working.
With the release of Mountain Lion on July 25, 2012, Apple continued to merge iOS and OS X. New to Mountain Lion was the Notification Center, a single place for apps to send alerts to the user, as well as the Game Center, which lets Mac and iOS users participate in common games.
Mountain Lion also changed application names to more closely resemble their iOS counterparts. Address Book became Contacts, iCal became Calendar, iChat became Messages, and Notes became a stand-alone app, instead of being part of Mail.
iCloud became more integrated into the OS, with the ability to store files directly from various applications in the cloud, without the need to mount any type of cloud-based drive or service.