Heh i see its been answered but yea its the thermal control for your heater valve. I had to change one of these as well on my old Police Interceptor spec merc last winter...That sucked harder then a cheap hooker having it just basically disintegrate on me at -30C when i was basically forced to change it for the same reason you had to change yours. If it works anything like the ford one basically it keeps the coolant circulating till it reaches a certain temp before sending the now hot coolant through the system. From looking at the pics this one appears to be vacuum operated. The fords simply acted as a trigger switch for the heater fan. Once it hit temp it would allow the fan to operate. Not sure exactly how the GM one works though lol...though im sure i will eventually learn...
