LDR A transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor
Phototransistor A light-sensitive transistor.
Preset variable resistor A preset is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment. Presets are cheaper than standard variable resistors so they are sometimes used in projects to reduce the cost.
Zener diode A zener diode can be used to maintain a fixed voltage.