It is just a tool to allow you to install different operating systems to run on top of your original.
With that, you can then run any apps that will run on these other operating systems.
I'm on a Mac. However I need to run MS. Office for work stuff.
I could purchase either Parallel or VMWare, but I chose the route of CrossOver.
With CrossOver, I don't need to install any of the base operating system and simply install MS. Office.
It gives the ability to run a lot of Windows Apps on my Mac.
http://www.codeweavers.com/