The history of OS X started with a bear, not a cat: in September of 2000, Apple presented Kodiak. For 30$, users could purchase this first public Beta of Mac OS 10.
The official OS 10.0, Cheetah, followed 6 months later. It featured Apple's brand new user interface Aqua and the first versions of apps like TextEdit, Preview, Mail, and QuickTime.
The new OS X demanded an (at that time: whopping!) 128 MB of RAM and 800 MB of disk space.
Just half a year later, Puma was released. Instead of delivering many new features, it focused on improving the system's performance.
With Mac OS 10.1.2, Apple also started shipping OS X by default with new computers - and replaced OS 9 for good.
Jaguar featured new applications like iChat and Address Book. It also introduced Universal Access, a component that made OS X more usable for people with visual and hearing impairment or physical disability.
Besides that, OS 10.2 showed a new startup screen: it replaced the Happy Mac logo with the new, fruity Apple icon.
Microsoft must have had mixed feelings with Panther: on the one hand, Internet Explorer for Mac was superseded by the new Safari as the default web browser. On the other hand, 10.3 brought many interoperability improvements with Microsoft Windows (including out-of-the-box support for Active Directory).
Additionally, Apple presented Exposé to improve working with multiple open applications.
Tiger brought the invaluable Spotlight search to Mac OS. It was also the first version to present Widgets (like a calculator, a calendar, or a clock) on the new Dashboard.
Besides that, Mac OS 10.4 inaugurated new hardware: it was installed both on the first Macs with Intel processors and on the first Apple TV devices (that were yet to come out later, in 2007).
No other Mac OS had been such a long time in the coming. However, Apple had been busy developing and releasing iOS and the iPhone in the meantime...
The new Leopard, anyhow, introduced a built-in backup system called Time Machine. It also brought full support for 64-bit applications.
Moreover, Mac computers could now run other operating systems like Microsoft Windows - thanks to Boot Camp.
Like the name suggests, Snow Leopard built mainly on top of its predecessor, Leopard. However, it introduced the new Mac App Store that was inspired by its successful companion on iOS.
Mac OS 10.6 also marked the end of the PowerPC architecture. Only Intel-based Macs were supported from this point on.
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The king of all cats, Lion, was the first version of Mac OS that wasn't available on CDs or DVDs anymore. The exclusively downloadable OS picked up many ideas from iOS: Gestures, Launchpad, window restoring, ...
Apple's newest part of infrastructure, iCloud, also debuted in Mac OS 10.7.
Mountain Lion brought even more successful concepts from iOS to the Mac: a new Messages app to replace iChat, the Reminders app, and the Notification Center.
Even before presenting OS 10.8, Apple made a big announcement: it committed to a yearly release cycle for new versions of Mac OS.
With Mavericks, Apple didn't only introduce a new naming scheme for its operating system. It also made OS 10.9 a free upgrade and promised to keep future updates free, too.
New applications like Maps and iBooks were also presented with Mavericks.
The biggest design change in years, Mac OS 10.10 follows its little brother iOS - with a flatter appearance and blurred transparency effects.
With the new Handoff functionality, users are able to move seamlessly between devices, even during individual tasks. Apple also emphasizes its Full Screen Mode feature: it's now triggered through the traditional green button in the window title bar.
El Capitan, named after a rock formation in Yosemite National Park, concentrates on performance & stability improvements.
Besides some smaller enhancements to Safari, Spotlight, Mail, and Notes, the introduction of Split Views is probably the biggest novelty: a way to conveniently arrange two applications' windows side-by-side.
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