From SKYbrary Wiki
A transformer is a device that allows AC voltage to be increased or decreased to the required value for transmission or for specific uses. It does this by transfering electrical energy from one circuit to another through inductively coupled conductors. The inductively coupled conductors are the windings or coils wound around a ferromagnetic core within the transformer. The alternating current in the first or primary winding creates a varying magnetic flux in the core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), or "voltage", in the secondary winding.
If a load is connected to the secondary, current will flow in the secondary winding, and electrical energy will be transferred from the primary circuit through the transformer to the load. The induced voltage in the secondary winding is proportional to the primary voltage and is determined by the ratio of the number of turns in the secondary winding to the number of turns in the primary winding. Thus, for a transformer to reduce or "step down" the voltage from 120V to 12V, the number of turns in the primary winding will be ten times greater than that in the secondary.