Virtualbox is a program that will allow you to run an operating system inside of a different operating system. For example if you use Machintosh OS X you can also run Windows within your OS X. It is a great program if you don't need to use many features of a different system, but only want to access a few things every now and then. I have used Machintosh and Windows mostly in the past. I have now switched to Linux and have not felt the need to access Windows much, so even though I have a dual boot of Windows and Ubuntu I find it much more useful to have Windows installed as a virtual machine though Virtualbox. I am able to just open Windows really quick and do something without having to log out of Ubuntu and then into Windows and then back again.
Virtualbox has two versions and some changes can be made to both and both are free. They have an open source version and a closed source one. I guess the main difference between the two that I have seen is that the open source one is not able to have the usb devices enabled, although I am not positive about that. The open source version will have the letters "OSE" in the name.
Virtualbox is available for download for Linux, OS X, Windows and Solaris. If you get the non-open source version you can get an extension pack to enable to the USB ports and also a "Software Developers Kit" (SDK).