In the most recent technologies, registers are virtual computers, sometimes using touch screens.
So essentially a virtual computer is a computing environment of some sort.
So that's sort of the broad brush background of virtual computers.
The System/360 was a virtual computer, a reference instruction set and capabilities that all machines in the family would support.
The virtual computer was developed in order to reduce the possibility of an unrecoverable software failure.
This is a program that lets you run multiple virtual "computers" in windows on your desktop.
And then you only have to implement that virtual computer that you've designed on the actual hardware.
EC2 allows users to rent virtual computers on which to run their own computer applications.
It can be used to provide virtual hosting of several virtual computers on a single computer.
Things might run a little more slowly if everybody is using their virtual computer at the same time.