LDR A transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor
An OR gate can have two or more inputs. The output of an OR gate is true when at least one of its inputs is true.
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.
A NAND gate can have two or more inputs. The o on the output means not showing that it is a Not AND gate. The output of a NAND gate is true unless all its inputs are true.