A software system that will mimic the installation, and operation of a complete operating system of choice by the installer. This is not always ideal as the primary OS must have resources that the virtual machine can not access. With that said, most VM software will require that you dedicate a number of CPUs, Memory, and storage capacity. There could be a number of reasons why someone will want a virtual machine, but it all centers around not getting a dedicated system for an operating system, and associated applications.