
Mac OS X was a radical departure from previous Macintosh operating systems; its underlying code base is completely different from previous versions. Its core, named Darwin, is a free and open source, Unix-like operating system (OS) built on top of the XNU kernel, with standard Unix facilities available from the command line interface. Apple layered over Darwin a number of components, including the Aqua interface and the Finder, to complete the GUI-based operating system which is Mac OS X.