Mac OS is the main operating system for the Apple Macintosh computer and Power PC.

It was the first operating system that featured a graphical user interface (GUI). However, the idea of having a GUI quickly propagated itself in other environments, like Microsoft's Windows OS.

Mac OS has an important market share, mainly due to its simplicity. Its advertising motto is "Just works". Many believe that this can be true because the Mac OS has only one type of hardware to worry about: Apple hardware. This leads to an increased stability and superior user experience.

Mac OS is based on BSD open source operating system kernels, that are available under the BSD license. Unlike the GNU license, the BSD license allows companies like Apple to take the code and embedded into commercial products. Many Mac OS software components are based on open source products.