Mac OS has a graphical user interface (GUI).
There could be several reasons. One likely one is that you do not have a modern version of Mac OS X. G3s shipped with Mac OS 8.6 or Mac OS 9. They cannot run Mac OS X 10.5 (Leopard) at all, and can only run Mac OS X 10.4 with hefty and usually unsupported upgrades. In the Related links section, you will find screenshots of Mac OS 8.6, Mac OS 9, Max OS X 10.0, and Mac OS X 10.1, so you can get an idea of what those systems look like, and if they look like the one you are running. There is also a link on how you can upgrade older Macs, although it is probably not worth the effort.
mac OS 9 is the previous Apple Macintosh Operating System version, before Mac OS 10, or X as it has been advertised as. Mac OS 9 does not utilize the 'aqua' look of the modern mac interface, but instead uses a chunky grey interface. http://www.iindigo3d.com/macosmacos9.jpg for a preview of the interface of mac OS 9
The main OS for Mac can be found on a Mac when it is switched on. The current main OS is called Mac OS X.
Mac OS 9 is the old version of Mac OS. Mac OS X was completely rebuilt from the ground up. Except some of the programs. The interface and look is different, but things like Disk Utility and stuff are the same. Terminal is a program that never gets improvements so that's the same too.
I have a Mac OS X but I know Mac OS is a computer made by the brand/company, Apple.
You cannot without erasing your hard drive and installing Mac OS 9 from scratch. Even then Mac OS 9 may not run on modern Mac hardware because Mac OS 9 needs the ROM chip to boot. Modern Mac OS X hardware uses EFI to boot the system instead of a ROM chip to boot. Mac OS X v10.0 - Mac OS X v10.4 support Mac OS 9 applications and can run them, you just need to install the Mac OS 9 compatibility program which can be found on the Mac OS X Install CD.
The second name of Mac OS X v10.4.5 is Mac OS X v10.4 Intel.Mac OS X 10.4 was known as Mac OS X Tiger.
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The oldest mac OS is System 1
Mac OS Leopard
Mac OS X 10.4 (Tiger) needs to be purchased as it is not possible to update from Mac OS X 10.2.8
Unless the disc is damaged and so needs reformatting you can run the Mac OS installer over an existing Mac OS.