Set of coils wound around a core, usually made of iron generating a magnetic field, that creates energy between the coils. Transformers step electrical currents up or down, and is the heart of computer power supplies. The illustration shows an example of the inside of a transformer. As you can see, the circuits are not connected, but are still able to transmit power over the electromagnetic field.
A transformer could step down incoming power by having more turns in the incoming or primary coil than in the secondary coil. For example, if there were double the amount of coil turns, it would reduce the power in half.