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.
Voltmeter Measures voltage. The proper name for voltage is potential difference but voltage is more widely used.
Electricity can flow only if there is a complete circuit from the battery through wires to components and back to the battery again.