A capacitor stores electric charge. It can be used with a resistor in a timing circuit, for smoothing a supply (it provides a reservoir of charge) and can be used as a filter (blocking DC signals but passing AC signals). Unpolarised capacitors usually have small values, less than 1µF.
Phototransistor A light-sensitive transistor.
Diode A device which allows current to flow in only one direction.
The diagram shows a simple circuit of a battery, wires, a switch and a lamp. The switch works by breaking the circuit.