VirtualBox is an open source software developed and maintained by Sun Microsystems. The main aim behind this kind of softwares is to provide a virtual environment for running guest operating systems. Which means you can run a different operating system inside your current operating system.
It is meant for those people who just love to experiment new things on their PC but are afraid of making any accidental damages to their system. You can also use the .vmdk image files which are created using the VMWare software.
If you are a first time user of virtual machines then you might find it a little difficult to use this software. You can read the step by step guide on creating virtual images and using them in VirtualBox over here at UbuntuGeek.
The important part to note here is the settings of this software. You can limit the amount of RAM to be used by this software. Set it to the 3/4th of what you have in your PC. So that you can run other softwares simultaneously without much trouble. By default when you click on the VirtualBox windows the mouse control is transferred to the guest OS. To use the mouse for the actual OS use the right ctrl key. You will learn more as you start using this software.
VirtualBox is an Open source software and is available for Windows, Linux and Mac systems. You can download the latest version of VirtualBox from the link given below.