The CDR(Current Dependent Resistor) is a refinement of the PTC thermistor. It's a relatively new component. The CDR has a more instant switch from low(a few ohms) to a high resistance(several kiloohms) than the PTC.

The CDR is manufactured for different 'switching' currents, and is used instead of a fuse. If the current through the CDR gets higher than the switching current the temperature in the component rises. This causes the resistance to rise rapidly, thous protecting the rest of the electronics in the circuit. When the temperature falls again, the resistance returns to the low state.