Mac OS Classic is a graphical operating system by Apple and the predecessor to the more modern Mac OS X. For questions about the latter, use the [osx] tag.
Mac OS Classic is a graphical operating system by Apple, used on their Macintosh line of personal computers from 1984 to 2000. Originally called simply System, it was renamed to Mac OS in version 7.6. Since 2001, it's been replaced by OS X, technically version 10 of Mac OS, but architecturally a major departure from its predecessor.
Mac OS was notable for differing from other contemporary operating systems by being completely graphical - it had no command-line interface. This was done in order to minimise the amount of technical know-how required to operate the computer. Actions that on other operating systems required accepting input from (or displaying information to) the user in the form of textual commands and prompts, were accomplished in Mac OS with graphical icons and mouse gestures. To provide such functionality at a low level (for instance, during boot time), portions of the operating system were kept in ROM on the motherboard. As a result, only computers manufactured or licensed by Apple were able to boot Mac OS.
Mac OS Classic initially ran on Motorola's 68000 series of microprocessors. Later versions added support for the PowerPC architecture as Apple started using it in Macintoshes. Early versions of OS X (up to 10.4) included a compatibility layer called Classic Environment that allowed Mac OS Classic applications to be run on PowerPC-based Macs.
