Light Emitting Diode A transducer which converts electrical energy to light. Usually abbreviated to LED.
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.
Resistor A resistor restricts the flow of charge. Uses include limiting the current passing through an LED, and slowly charging a capacitor in a timing circuit. Some publications use the old resistor symbol
LDR A transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor