Each tile gets a unique, relatively straightforward name/ID, and then you can map this name/ID to a specific lat/long (with the accuracy being determined by the zoom level you use)
I know there are tiling schemes used by Google Maps, Bing Maps, etc, but I seem to recall there was something unique in the approach of this website.
Unfortunately, my web search has been fruitless and I'm not able to find the relevant HN thread.
Do any of you remember this?