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.
Light Emitting Diode A transducer which converts electrical energy to light. Usually abbreviated to LED.
Photodiode A light-sensitive diode.
Diode A device which allows current to flow in only one direction.