What exactly is a foreign OTC stock?

Take SPHDF for example. It's related to a stock on the Swiss stock exchange, SANN:SW.

If I buy shares of SPHDF, what am I buying?

Is this some kind of proxy, or are shares from the foreign exchange being sold on an American market? Is it even being bought and sold on an American market? What is the relationship between the price of SPHDF and SANN:SW?