VMOS stands for Virtual Machine Operating System. The concept of this is that you use one version of your Windows operating system, install it in a virtual machine, and then use the virtual machine to run a virtual Windows application on the device. So for example if you have Windows XP, you would install the XP operating system and a virtual machine called "Windows XP". Then you would have two different versions of the same Windows operating system, "Vista"Virtual Vista", which is both running on the device. You can then run two different Windows applications (either on Windows XP or on Virtual Vista).
Some people think that these types of programs are not free because you cannot get the full program, you only get a version of it's free, but there are many free versions. For example there is a free version of XP. Then there is also a free version of Windows Vista, and if you want the "full" version of the program you just need to pay for it.
And this brings us to another point, most people think that since the program is free, they can use it for any kind of reason they want, as long as they are over the age of 13. Well you cannot do this with the free versions because the law prohibits this. However it is perfectly legal to use the software for commercial purposes. And so it can be used for building your own home security system, as a web server, as a backup server, for playing online games, for monitoring your home network, etc.