SSD= solid state drive, on many new computers they either install hybrid hard drive or have a small dedicated SSD to improve on booting time and to place cache of frequently used programs.
Often these drives are not user accessible (unless you are willing to deal with lower level system controls that is)
And thus if they made this drive as your first active boot drive, you could not change boot order to another OS other than the OS that is already appointed.
ie. if you cannot add another boot device unless you can re appoint another OS starting location.
VM = Virtual machine. Eg, VMWare, Virtual PC etc..
Since most program are not portable to another type of OS, there exist programs called Virtual machines, where they EMULATE another OS within the base OS.
So for example, you can have full complete Red hat, Ubuntu, Mac OSX, WIN XP, WIN 2000, Win 7 and Darwin, running simultaneously and independently within your base system.
VM is very useful since the actual installation can be portable and switched on as needed, eg, if you need to run specific program on Linux, yet your computer runs windows, simply run VM and open your Linux OS image (within your windows environment) and run the program.. Vice versa it can be Linux as base and runs Windows in VM.
My main desktop computer has Ubuntu as a base and I run windows on Virtual PC as needed, and I plan on my new laptop to run Ubuntu on VM with Win 7 as a base.
Elementary.. of course we know that..after all this is a hunting website ..