VirtualBox is a professional-grade, open-source virtualization software. If you're not happy with the virtualization options available to you in the paid software marketplace, then exploring VirtualBox might be for you.
Range of supported systems: VirtualBox is available on all the major operating systems. This allows you to take advantage of open-source virtualization whether your platform is Mac, Windows, Linux, or Solaris. You can then run most versions of Windows, DOS, Linux, or Solaris as a virtual system.
Lots of info: One of the typical advantages of open-source software is that the user community creates a wide range of help and technical documents to assist people in the software's operation. VirtualBox is no exception, and there is ample documentation available to anyone who needs help with their VirtualBox setup.
Operating system integration: The paid virtualization solutions have much better integration between the host system and the guest operating system. For instance, using VMWare or Parallels, you can drag and drop files from one system to the other. You can't do that with VirtualBox.
Lack of active support: Virtualization software is complicated and can easily have things go wrong with it. When this happens with a product that you're paying for, you have access to a lot of hands-on technical support. That doesn't exist with open-source products. If you run into a conflict, you'll be stuck trying to sort it out on your own.
If you're highly technical and capable of using online resources to troubleshoot and configure your own software, then VirtualBox could work for you. On the other hand, if you need a fully-packaged solution with good support, VirtualBox might be too tricky.