But what does this do exactly, like I asked in the thread title. Im unfamiliar with "virtualization" Ive never seen this live. Will my Mac operate like if it had a true Snow Leopard install? (ive no plans of ever using mountain lion). It looks like the OP on the thread you linked expressed concerns about using Time Machine/keeping any important files on the snow leopard in virtualization.
Virtualisation is the running of one OS within another OS as a virtual machine instead of a real machine.
Let's say you have a an iMac with a quad core CPU and 8 GB RAM and running with
OS X 10.8 Mountain Lion. This is the real machine.
Then you want to install
Mac OS X 10.6 Snow Leopard onto it, but in order to do so, you choose between VMWare Fusion and Parallels Desktop as the two main choices for running virtual machines on Mac OS X.
You then open VMWare Fusion / Parallels Desktop and install the OS of your liking, in your case,
Mac OS X 10.6 Snow Leopard.
It will be stored as one or several files on the HDD/SSD of your choosing.
Once installed, you can give that virtual machine resources, as you have a real machine with four cores and 8 GB RAM, you can give the virtual machine half of that, two cores and 4 GB RAM, thus you have still two cores for everything outside this virtual machine and 4 GB.
Therefore you can run two or more OSs at the same time, the one using the real machine,
OS X 10.8 Mountain Lion in this case, will be faster, since there are no additional layers between the OS and the hardware like with a virtual machine (OS and VMWare Fusion / Parallels Desktop between virtual OS and real hardware).
Anyway, you cannot run a virtual machine when Mac OS X is not booted.
It is not the best of explanations, but maybe have a look here, even if it pertains to Windows instead of Mac OS X, the same principles apply:
Booting Windows on the Mac