Emulation
Your first choice is emulators. Emulation is the art of making one computer (in this case your Mac) behave like another.
The best place to go to find out everything about emulation on the Mac is Emulation.net. If the Mac can emulate it, it is listed here. Everything from old arcade machines to full Wintel PC's. You can even emulate the original Apple's and Macs, perfect for that old game you've been pining after.
With products such as Real PC and Virtual PC that emulate the entire PC hardware on your Mac, it is even possible to run any OS that runs on wintel PC's.
Emulations run as normal Macintosh applications, so you get the ability to run another operating system and its applications without losing the MacOS. On the other hand, since you are emulating the other machine, your Mac has to do the extra work of first interpreting what the emulated machine should be doing, and then replicating it on your Mac. This means that emulators are slow (a good rule of thumb is about 50 per cent the speed of your Mac).
Porting operating systems to the Mac
Some operating systems are written so that they can be run on many different platforms. A good example of this is Unix. Each different manufacture of Unix machines uses different hardware in there boxes. I'm not talking just different hard drives and CD-ROM's, I'm talking about the complete machine. They don't even use the same chips. IBM uses PowerPC chips, Sun uses Spark chips, Hewlett Packard, Digital, NEC they all use chips designed in-house specifically for there own computers, yet they all use Unix as their operating system. Unix is designed so that the operating system can be easily ported to different hardware.
Ported operating systems run at their full speed. There is no slow down due to instruction translation like in emulation. Unfortunately, due to the differences in hardware, programs for operating systems that are ported are usually distributed as source code (the actual programming code in a programming language such as C, C++, PASCAL, etc). The end user then has to compile the code (change it from source code to a binary file the computer can read and understand) so that it can run on his machine.
