Problem: "I don't know if I have System 7, OS 9, OS X, or what. How do I find out which Macintosh operating system I have?"

Solution: Up in the top-left corner of the screen, you have a little Apple logo. This is the Apple Menu. If it's rainbow-colored, you're in OS 9 or earlier. If it's blue or gray, you have some version of OS X ("X" is for "ten", like the roman numeral). Click on that Apple to open the Apple menu, and choose the first item on the list: it will be About This Mac, or About This Computer, or About This Macintosh. A window will open telling you what version you have. That's it!

Not at the computer? You might still be able to figure it out. You might know the nickname of your operating system, for example, Panther. If it's a big cat name, then you have some version of OS X.
Cheetah is 10.0,
Puma is 10.1,
Jaguar is 10.2,
Panther is 10.3,
Tiger is 10.4.

If your computer was made before March 2001, it has OS 9 or earlier unless you've bought and installed OS X. If your computer was made before October 1999, it has System 8 unless you've bought and installed a later version. If your computer was made before January 1998, it has System 7 unless you've bought and installed a later version. If your computer was made before 1991, then you had better hope it's been upgraded to System 7, or you'll have a real hard time getting on the internet.

If you have bought a used Mac computer and it won't start up and you want to know which version of Mac OS it's compatible with so you can install it and get the computer working, download Mactracker (onto a working computer, of course) and look up your model.