In many vehicles, the temperature blend door controls how much air is diverted past the heating core before coming through the various vents in your vehicle. In old cars, you would often manually move a lever on your dash, which would connect to a cable that opened and closed the blend door. This was how you controlled temperature. In modern vehicles, this is often done through computer controlled electronics. As you turn the heater knob on your Equinox, the computer system sends signals to an actuator that opens or closes the blend door to provide the correct mix of hot and cold air.
Unfortunately for Equinox owners, the actuator often tries to push the blend door past the point of being closed. Over time the continued stress on the blend door can lead one or more of the involved parts to fail. Either the blend door itself can break, or one of the parts of the actuator can fail. With the inability to move the blend door, the Equinox is now stuck on whatever heat/cooling setting it was on at the point of failure. The result in my case (and that of thousands of others) was that the vehicle could not receive any heat through its vents. No heat in a Chevy Equinox means no defrost, impaired visibility, and dangerous driving.
More information (http://wiki.answers.com/Q/How_does_a_blend_door_work)