That's actually not quite true. In digital circuits, we do often use pull-ups or pull-downs to keep an input from floating. That's not what's happening here.

That's actually not quite true. In digital circuits, we do often use pull-ups or pull-downs to keep an input from floating. That's not what's happening here. You're making a voltage-divider. One half of the voltage-divider is your temperature sensing resistor (thermistor), and the other half is the resistor you're asking about. Go look up voltage-dividers. In the meantime, I made a little javascript helper app as one of my very first 'learn javascript' apps a while ago. Check out http://eebug.com http://eebug.com to play with a voltage divider. In the app, both legs of the voltage divider are constant resistors, while in this circuit, one leg's resistance varies with temperature, but it's the same concept.