OS X is Apple's operating system that runs on Macintosh computers. It was first released in 2001 and over the next few years replaced Mac OS 9 (also known as Mac OS Classic) as the standard OS for Macs. It was called "Mac OS X" until version OS X 10.8, when Apple dropped "Mac" from the name.

OS X was originally built from NeXTSTEP, an operating system designed by NeXT, which Apple acquired when Steve Jobs returned to Apple in 1997. Like NeXTSTEP, OS X is based on Unix and uses the same Mach kernel. This kernel provides OS X with better multithreading capabilities and improved memory management compared to Mac OS Classic. While the change forced Mac developers to rewrite their software programs, it provided necessary performance improvements and scalability for future generations of Macs.

The OS X desktop interface is called the Finder and includes several standard features. OS X does not have a task bar like Windows, but instead includes a menu bar, which is fixed at the top of the screen. The menu bar options change depending on what application is currently running and is only hidden when full screen mode is enabled. The Finder also includes a Dock, which is displayed by default on the bottom of the screen. The Dock provides easy one-click access to frequently used applications and files. The Finder also displays a user-selectable desktop background that serves as a backdrop for icons and open windows.

When you start up a Mac, OS X loads automatically. It serves as the fundamental user interface, but also works behind the scenes, managing processes and applications. For example, when you double-click an application icon, OS X launches the corresponding program and provides memory to the application while it is running. It reallocates memory as necessary and frees up used memory when an application is quit. OS X also includes an extensive API, or library of functions, that developers can use when writing Mac programs.

While the OS X interface remains similar to the original version released in 2001, it has gone through several updates, which have each added numerous new features to the operating system. Below is a list of the different versions of OS X, along with their code names.

Updated December 31, 2014 by Per C.

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