Virtual machines are computers that run inside other computers but appear to be separate computers. Virtual machines are getting popular now.
There are many reasons for using virtual machines
First they are used where there used to be many servers and now with the processing power and memory capabilities and sizes of hard drives it is no longer needed to have separate physical servers. Now by creating a virtual server inside a server they still appear to be two or more different servers as we had been running but only one physical server. Reduces the need for space and also reduces our power needs both for the servers and air conditioning.
Secondly you can set up different machines on your desktop and keep functions within a certain virtual machine and have no effect on the other machines when a machine has problems. My networking students set up virtual machines on their home machines to install and work with Windows 7 and Windows Server 2008. I have a virtual machine on my desktop to play with Windows 8 and will soon set up pone to play with Windows 10.
Popular programs to do virtual machines include Virtual PC, Virtual Box, Hyper-V, VMWare and Virtual Server