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.

Classic MacOS: 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 MacOS: 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

MacOS X: The Current
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

Will need some more info!