MacOS

Mac OS is the old version of Mac OS X which is the newer name for System. It had many versions which looked pretty much the same, but mainly feature and performance upgrades. Mac OS X only discontinued Mac OS when it's third release came out.

Versions
Mac OS has seen many different versions but mainly there the same. If you want to look at older than 7.6 look at System

Mac OS 7.6
Mac OS 7.6 is the first major release with the "Mac OS" prefix in it. It's codename was "Harmony" and it has quite a few features including a revamped "Extensions Manager", more native code for PowerPC, more Internet tools and a more stable Finder. The only feature removed was PowerTalk as it didn't have much support at all.

Mac OS 8
Mac OS 8 is a big major release, as it brought about many new and improved features including a lot of modernizing of the Mac OS system. One of the major things is that Apple began porting everything over to PowerPC based applications. Mac OS 8.1 had a new file system known as HFS Plus. Then 8.5 came which required the computer to have a PowerPC processor.

Mac OS 9
Mac OS 9 is the last of the Mac OS releases, proving to make it as good as it was going to get. It included a lot of Internet functionality. This is when Mac OS X was introduced and dammed the end of Mac OS as it was far too old to live on.