The diagram shows a simple circuit of a battery, wires, a switch and a lamp. The switch works by breaking the circuit.
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.
Loudspeaker A transducer which converts electrical energy to sound.
Galvanometer A very sensitive meter used to measure tiny currents, usually 1mA or less.