VirtualBox is an open-source and multi-platform tool, available for Windows, Linux, and Mac OS X, as well as other operating systems, that lets you create virtual disc units where you can install a guest operating system, inside the one on your own computer, and use it as if it were actually installed.
The virtual disc where the system runs is completely customizable, and it lets you modify the virtual hardware to whatever specs you need, be it the processor, the RAM memory, or the storage capacity. You have to take into account that these take part of the actual physical resources of your computer, so you need an equipment that is powerful enough to run both the guest and host operating systems.
The program supports almost all versions of the most recent operating systems, such as Windows 10, Mac OS X Yosemite, and the latest updates of Ubuntu or any other Linux distribution. The advantages of creating virtual units are many, like using programs that are only compatible with one operating system or the other, taking the virtual modular units with you, and creating safety copies of them. In fact, an interesting feature in VirtualBox can save sessions on the operating system running on the virtual unit, so you can close it and come back exactly where you left it.