An energy-dense polarized capacitor type, used in low-frequency filtering applications.
Electrolytic capacitors are commonly fabricated from strips of metal foil (often aluminum) separated from itself by paper insulator layers. The entire assembly is coiled up and placed into a metal can, which is then filled by a liquid or polymer electrolyte. As part of the fabrication process, one foil will form an aluminum oxide layer - this foil will act as the anode, the other foil (along with the electrolyte) the cathode. The aluminum oxide layer will ultimately be the dielectric barrier between the foils (the paper just provides some insulation until the oxide layer is formed).
Due to the metal foil construction, electrolytic capacitors tend to have much larger equivalent series resistance (ESR) and equivalent series inductance (ESL) than capacitors of other constructions (like ceramic). However, the large surface areas allow for extremely high capacitance values. They are commonly used in power supply circuits, at frequencies from DC up to a few hundred kilohertz.
Electrolytic capacitors are polarized and can fail if reverse voltage is applied. They can also fail due to electrolyte evaporation, which is accelerated by high-temperature operation.
During long periods of storage, the oxide layer may dissipate, reducing the functionality of the capacitor. Such a capacitor will require forming, in which voltage on the capacitor is slowly increased, after which it is allowed to hold at full voltage for some minutes or hours. Sudden application of voltage to a capacitor requiring forming can result in catastrophic failure.
Wikipedia article on electrolytic capacitors
