Quadkeys are Bing Maps's way of subdividing a Web Mercator map of the world into tiles. It is a simple quadtree-like system where each tile at a spcific zoom level has exactly four children:
I find myself needing to convert map location on earth in longitude / latitude to a Bing quadkey sometimes, and I found there is no easy web tool to do this, so I wrote one: What the Quad:
As a bonus, it will also give you the z/x/y coordinates, a different hierarchical tile indexing system popularized by OpenStreetMap. The OSM wiki has a decent description of this system.