Electrolytic capacitors have a metallic anode covered with an oxidized layer used as dielectric. The second electrode is a non-solid (wet) or solid electrolyte. Electrolytic capacitors are polarized. Three families are available, categorized according to their dielectric.
Aluminum electrolytic capacitors with aluminum oxide as dielectric
Tantalum electrolytic capacitors with tantalum pentoxide as dielectric
Niobium electrolytic capacitors with niobium pentoxide as dielectric.