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.
Voltmeter Measures voltage. The proper name for voltage is potential difference but voltage is more widely used.
Transistor NPN A transistor amplifies current and can be used with other components to make an amplifier or switching circuit. This symbol is for a bipolar junction transistor (BJT), the type you are most likely to use at first.
LDR A transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor