An inductor is a coil of wire. Inductors have both ac and dc resistance. ac resistance is known as impedance. The impedance of an inductor is much greater than the inductor's dc resistance.
When electrons flow through a wire, they produce an electro-magnetic field, abbreviated emf. In a coil, the turns of wire will produce an emf field that will slightly impede, or resist electron flow of a dc current. When an ac current is used, the ac impedance is very high, compared to the dc resistance. This is because the ac signal is constantly creating an electro magnetic field that is produced has a polarity on it. An ac signal starts at zero volts then begins to climb to its full voltage. At the point that the ac signal reaches its peak voltage, a magnetic field has been built up in the inductor. As the ac signal voltage begins to fall to zero volts, this built up magnetic field begins to pull electrons through the wire, which is known as the generator effect. When the ac signal crosses the zero volt line and begins to conduct in the opposite direction, the magnetic field is trying to pull the electrons the opposite direction. This creates a lot of ac impedance.
A common electronics term is, the current leads and the voltage lags n an inductor, abbreviated, ELI. This means that when viewing the sine waves for the voltage and the current of an ac signal through an inductor, the voltage level will be out of phase with the current, on the time divisions, it will be a few steps ahead of the current sine wave. The current will follow what the voltage does, but will lag behind it, because it takes work to create a magnetic field.
Inductors are used as filters, transformers, and resonators. In a transformer, two coils are wound onto the same metal conductor, and the voltage and current from one coil is transferred to the other coil. By varying the number of turns on the secondary coil, voltage can be stepped up or down. Inductors are often used with capacitors because capacitors have an opposite effect; current leads and voltage lags in a capacitor. In a tuner circuit and in class C amplifiers, a capacitor will be connected in parallel with an inductor, and will produce a tank circuit that will resonate at a specific frequency.
Basically, there are air core inductors, and inductors with a conductor for a core. The conductor can be iron, ferrite, or some other type of metal.