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.
Mac OS X has different particles unlike Windows, were everything is a bit simpler then a Mac OS X.
The second one has a lowercase 'i' in front of it.
No. An iMac is a computer from Apple. Mac OS X is an Operating System for Mac computers. Every Mac comes with a copy of Mac OS X - pre installed.
There are apps that will only work with Mac OS X which can be on any Mac computer - iMac, MacBook, Mac Mini etc.
Mac OS X 10.6 (Snow Leopard) only works with Macs with an Intel processor. The last version that will work with a G5 Mac is 10.5 (Leopard).
There are no viruses affecting Mac OS X.
Holding down the Option key (alt) while the Mac is starting up will offer you the choice of how you want to boot the Mac. Selecting Mac OS X will make that the default boot.
To upgrade beyond Mac OS X 10.4 (Tiger) you will need to purchase Mac OS X 10.5 (Leopard). If the iMac has an Intel processor you can jump straight to Mac OS X 10.6 (Snow Leopard) but if it has the older G4 or G5 processor then 10.5 will be as far as it can go.
No. You can only play games made for Mac OS on an iMac.
Yes, you can upgrade to Lion from any iMac that uses the Intel processor. But the newest OS is Mountain Lion.
You can not legally run Mac OS on any computer not designed by apple.
The G5 came with the OS 9 operating system and had a Power PC processor. It was the last edition of Mac OS. The iMac switched to the OSx operating system with the next iMac. There was an education only model G5 that did ship with OSx Panther.
Unix is a specification. Mac OS X is a certified implementation of Unix. This like asking "What is the difference between a turtle and a red-eared slider."
The iMac G4 is able to run any version of Mac OS X up to and including, version 10.5 (Leopard).