That would be a "pull down" resistor, a "pull up" resistor has the other end attached to a positive voltage source. A "pull down" resistor would be used with a positive triggered circuit, a "pull up" resistor is used with a negatove triggered circuit.

What these do, is force the circuit to be at the the "rest state potential". I other words, it causes a positive triggered circuit to be pulled down to ground, when the circuit is not triggered to be on. The eliminates any "floating" of the circuit that can happen.