An electrolytic capacitor contains two sheets of aluminium foil with one covered in oxide. Thsi forms a dielectric between the foils and insulates them from one another. The thickness of the oxide determines both the capacity and the voltage withstand of the capacitor. Also the capacitor is polarised ie the positive voltage must be applied to the terminal marked + and the negative voltage to the other marked -. Liquid such as a solution of Ammonium Phosphate between the foils maintains the dielectric and completes the circuit between foils.
A rubber bung in the top of the capacitor will pop out and relieve any large build up of generated gas. This usually indicates the end of the capacitor's useful life.
This capacitor has a very low ESR (Equivalent Series Resistance), which suits it to handling large varying currents (measured in Amperes) without overheating, when the impressed Voltage changes.