They may be wound on tubular FORMERS or may be self supporting.
The former may contain a metallic core up its centre.
Iron cores are used for frequencies below about 100 kHz.
Ferrite cores are used for frequencies up to say, 10 Mhz.
Above 100Mhz the core is usually air and the coil is self supporting.

At low frequencies the inductor may have hundreds of turns, above 1 Mhz only a few turns.

Most inductors have a low DC resistance since they are wound from copper wire.

Inductor values of INDUCTANCE are measured in HENRIES.

Inductors oppose the flow of ac current.
This opposition is called INDUCTIVE REACTANCE.
Reactance increases with frequency and as the value of the inductance increases.