Inductors are devices that use counterelectromotive force (CEMF) to oppose changes in voltage applied to the inductor. Inductors are made up of a coil of wire occationally with an iron or ferrite core. If a voltage is applied to an inductor, it will build up a magnetic field that moves outward, and cuts through the wires of the coil and produce a voltage in the inductor opposite in polarity to that of the applied voltage, called CEMF. If the voltage is then removed, the magnetic field will then collapse, and as it does, cut through the wires of the inductor and produce a voltage in the direction of the voltage that was once applied (it is still CEMF because it is opposing the change in voltage).
Inductors are measured in units called henrys. One henry is the amount an inductor will have if it produces one volt of CEMF when the change of current applied is one ampere per second.