Issues are addressed in various components, including the virtual machine manager, 3D support, networking, USB, power management, virtual hard disk support, and the Linux additions.(Additions provide improved integration with the guest operating system.)
There are also specific fixes concerning issues with Linux and Mac OS X hosts.
The only non-bugfix item mentioned in the changelog for version 3.1.6 is that the vboxwebsrv web service is now multithreaded.
This is the second maintenance release of VirtualBox this year.
The full version of VirtualBox is released under an unusual licence that permits ongoing use at no charge providing it is installed by the actual user. Corporate users are expected to buy licences. A slightly cut-down version (no RDP or USB support) is available as source code only under the GPL.