>Can anybody give me a very high level overview of how VM works? For example, my customers often create a VM for my application. Does it mean that for each VM they have a separate box (PC)? Or more than one VM can be "installed" or "reside" on a physical box?
>
>Thank you.

Note: My experience so far is only with VMWare.

A VM (virtual machine) is an emulation of a computer + operating system, within a Windows (or other) installation. A file is reserved for the virtual machine; for the virtual machine, this file looks like the hard disk. In the case of VMWare, I might define a hard disk of 100 GB, but with the initial installation (only Windows), only about 1 GB is actually used. In other words, the file that represents the hard disk grows as needed (at least, that's one of the options. It is also possible to reserve the entire space from the start; pressumably you might want to do this for efficiency reasons.) It is possible to clone a virtual machine, so I don't need to install the operating system in the VM every time, for example; also, it is easy to create points in time, to which I can return later.

The programs that run in the VM are quite isolated from the host machine. You can install programs in the VM, while the VM is shut down, these won't affect the host machine in any way, except for the space used for the VM. However, it is quite simple to copy and paste files between the host machine and the virtual machine.

With respect to your specific questions: You can install several VMs on a host machine; each can have a different operating systems; I believe you can even run more than one at a time, if your computer has enough resources.