Electrolytic capacitors are called as one of the plates of the capacitor is made of an ionic conducting liquid, an electrolyte. These capacitors must be connected in a fixed polarity. Hence, these capacitors cannot be used in AC circuits without a dc bias. These capacitors have a high capacitance value.
Polarity is usually indicated in the capacitors with the positive lead longer than the negative lead. Alternatively, the polarity markings are made in the capacitor body.
Connecting these capacitors in the wrong polarity will cause heating of the electrolyte and lead to an explosion, a catastrophic failure. Most capacitors are provided with a vent to relieve pressure and prevent explosions.