Your car has a solenoid mounted on the starter. The key switch supplies current (through the starter safety switch on the shift linkage if automatic transmission) to the solenoid. The solenoid both engages the starter pinion gear and supplies electricity to the starter motor. You probably have a bad connection or a bad battery. The click you hear is probably the starter solenoid engaging the pinion gear, and closing the electrical contacts which supply electricity to the starter motor. If the electrical current available is inadequate to turn the starter motor, all you will get is the click.