An inductor is a passive electrical component that stores energy in a magnetic field when electric current passes through it. There are three main types: air core, ferromagnetic core, and variable inductors. Inductance is a measure of how much energy is stored in the magnetic field created by current flowing through the inductor. Self-inductance refers to the property of an inductor to store energy in its own magnetic field, while mutual inductance describes the magnetic field induced in one coil by another coil. Inductors are used in electric circuits, especially together with capacitors and resistors to create filters and for signal processing. They are also used in inductive sensors.