With virtualization you can literally run two, three or more operating systems on the same computer at the same time without the need for any redundant hardware or restarting. What this means for the end users, especially those operating in the Mac sphere, is that you can now simply install all the other operating systems and applications you may need without having to buy more than one computer.
What you will need to get started with virtualization is any new, meaning Intel based, Mac computer or any Wintel PC with an Intel processor with VT technology and enough Random Access Memory (RAM) for at least two operating systems, host and guest, to have at least 512 mega bytes available to each of them at the same time.
The RAM amounts aren‚Äôt necessarily required but if you plan for example to run Windows XP and Mac OS X at the same time, then in order for the performance to be good in both operating systems, your computer should have RAM at or in excess of one gigabyte. A good guideline is that you need as much RAM as would make the operating systems you intend to run in parallel each happy at their minimum RAM needs and then allow your guest operating systems to have access to a larger chuck for running their applications. It's probably important to note here, so I will, that a guest operating system will not consume the entire block of RAM that you allocate to it, during initial set up, while running but not doing anything. In other words just like applications in your host operating system a guest operating system will only use RAM when it needs it leaving it free for the host and other guest operating systems until needed.
Once you have all Virtualization hardware requirements met you will need software to get the whole thing running. Two commercial Mac and PC compatible options can be found at: http://www.vmware.com and http://www.parallels.com and if your more of the open source do-it-yourselfer type then you can check out: http://fabrice.bellard.free.fr or for Macs specifically http://www.kju-app.org/kju/ Then the only thing left to do is pick out and install your guest operating system of choice.