MacOS

macOS is a series of operating systems developed by Apple for their Mac platform. macOS is the second-most commonly used operating system after Microsoft Windows. It began in 1984 with the release of the Macintosh 128k. For a build list see Mac OS Buildlist.

The classic Mac OS originally used the 68k architecture from 1984 to 1991, then PowerPC from 1991 to 1999. Classic Mac OS was based on a simple kernel architecture, and eventually became too limited. In response, Apple bought NeXT and used that base to develop Mac OS X. Mac OS X continued PowerPC support until 2006, when Macs started shipping with x86 (Intel) processors. Support for PowerPC in Mac OS X was dropped in 2009.

PearPC is a useful emulator that can run PowerPC Mac OS under Windows or other x86 operating systems.

Mac OS X / macOS
Mac OS X is the current incarnation of Mac OS. The first version released was Mac OS X Server in 1999. Unlike the classic Mac OS, Mac OS X is based on NeXTSTEP (OPENSTEP) and uses its frameworks and architecture.