phinnaeusG-Sync is a module inside the monitor. It's actual hardware. It syncs up your monitors refresh rate to the frames output by the GPU, which is the opposite effect of Vertical Sync. This cures screen tearing and input lag.
Here is an article: http://www.anandtech.com/show/7582/nvidia-gsync-review
Here is an article: http://www.anandtech.com/show/7582/nvidia-gsync-review