VirtualBox is a VMWare equivalent, where it allows you to start a process that acts like a separate machine hosted in your operating system. You'd download it, install an OS image into the virtual machine, and run it; this allows you to use one machine to act like two (like you're running Linux right alongside your Windows machine or whatever.)
Developers use this sort of arrangement for testing, or development (for example, there's a Java development image with JavaFX, JDK 1.6, Netbeans 6.9, and Glassfish, although it's for VBox 3.2, instead of VBox 4. VirtualBox 4 won't load it, I tried and couldn't see where .vmdk files were supported.) This sort of thing is also found in operations centers; people buy monster rack-mounted machines like this Cisco UCS device, and then install virtual machines onto that.
This allows you to run a private cloud, for example, and you get isolation between nodes in the cloud (in addition to being able to use different OSes, so if Windows has a feature you need, you can run it right alongside your Linux box.) The key here is to make sure the rack machine you get doesn't suck.
This sort of deployment architecture is what companies like VMWare use to make their money, enough that they could afford to eat SpringSource. I wonder if Oracle was thinking of doing the same sort of thing - it's really big business lately, from the amount of coverage the concept gets and the activity of companies who use the "cloud" as marketing.
So this update is a good thing - VirtualBox is free and open source, unlike some of its competitors, and it looks really pretty nice.