The VirtualBox Extension Pack adds useful new features to this popular virtualisation package.
It includes a USB (EHCI) controller, for instance, which should allow you to get better performance from your USB devices. You'll probably need to enable the controller manually, though, as well as add filters for the USB devices you'd like to use (the VirtualBox manual has more details).
You also get VirtualBox Remote Desktop Protocol (VDRP) support. Essentially this allows you to run a virtual machine on one PC, while you view and control it from another: again, the manual has the full story.
And there's also support for remote booting a computer through emulation of the Intel PXE boot ROM with support for the E1000 network card.
None of this requires separate installation, as the extension pack has a "vbox-extpack" file extension that's handled by VirtualBox. So close down VirtualBox, download and double-click on the extension pack, and VirtualBox will launch and install it for you, upgrading any earlier version you might have.
The Extension Pack enhances VirtualBox with some useful capabilities.