A capacitor is a device that stores electric charge between two conductors separated by an insulator. There are different types of capacitors including non-polarized capacitors that can be connected either way in a circuit, polarized capacitors that must be connected with the correct polarity, variable capacitors whose capacitance can be varied by moving plates, and trimmer capacitors designed to be adjusted once and then left without further adjustment. Capacitors are used in timing circuits, as filters to block DC signals, and in radio tuners.