This is an article posted in Appstorm
Every computer needs an operating system to operate, just like we humans need our brains to function. Unlike us, computers can have more than one brain, running multiple operating systems at the same time. Virtualization is the process of concurrently running another (fully functional) operating system over the main OS X installation.
The great advantage of a virtual machine is that your original system is untouched – you can operate or remove a Windows installation without causing any harm to OS X. This how-to will walk you through the process of setting up Windows on your Mac using the free VirtualBox application.