Virtual Machine versus Computer Cluster

Virtual machine is a machine that does not exist physically but from the perspective of the one who use it, this machine is the real one. How can we do that? There is a technology so called virtualization that allows in one machine to be installed more than one operating system. You can find how to do this from the other posting. What I want to write here is the background of virtualization. Why do we need virtual machine? See, every computer include a server, most of their time they will be in idle state, let's say only 10% of the cpu capability they are using except for cpu intensive application. To increase that utilization of cpu we make a virtual machine on a real machine, for instance in one machine we can make four virtual machine. If it is linear we can say the number of utilization of cpu became 40%.

In the other hand, for a program such cpu intensive application, we cannot buy such machine that has very powerful processor/machine. Rather than buying a new one which is so expensive we can use the old machine to overcome that problem. How? We can make four old machine as a cluster, from the perspective of the user they see that this is one machine which is very strong. How to do that, see in the other posting in this blog.

The point is that the background of the two things is antipodes. So how can we make virtual cluster? What is the background of it?