MacOS

Mac OS is the second-most commonly used operating system after Microsoft Windows. It began in 1984 with the release of the first 128k Macintosh. For a build list see Mac OS Buildlist. Mac OS originally used the 68k architecture from 1984 to 1991, then PowerPC from 1991 to 2006 when it switched to x86 (Intel). PearPC is a useful emulator that can run PowerPC Mac OS under Windows or other x86 operating systems.

Classic Mac OS: The Original
This version lasted from version 1.0 in 1984 to version 7.5.5 in 1994. MacOS at this point was in fact known as System Software.

Classic Mac OS: The Somewhat-New
7.6 in 1995 was the first version to call itself MacOS rather than System Software, this was followed by version 8 in 1997 and 9 in 1999 (with version 9.2.2 released in Dec. 2001 as the final update to classic MacOS. For more information see MacOS 7.6, MacOS 8 and MacOS 9

Mac OS X
Version 10 or MacOS X is the current incarnation of MacOS. Its versions started with MacOS X Server in 1999 and Rhapsody in 1999 which were both used for servers. Unlike Classic MacOS, X is based on Unix and uses its frameworks (along with that of the X Window System for optimal effect.) Its versions are: Public Beta (Kodiak) September 2000

10.0 (Cheetah) March 2001

10.1 (Puma) September 2001

10.2 (Jaguar) August 2002

10.3 (Panther) October 2003

10.4 (Tiger) April 2005

10.5 (Leopard) October 2007

10.6 (Snow Leopard) August 2009

10.7 (Lion) July 2011

10.8 (Mountain Lion) July 2012

10.9 (Mavericks) October 2013

10.10 (Yosemite) October 2014