Set of coils wound around a core, usually made of iron generating a magnetic field, that creates energy between the coils. Transformers are used to step-up or step-down electrical currents and is the heart of computer power supplies. In the picture to the right, is an example of what the inside of a transformer may look like. As can be seen in this picture, the circuits are not connected but are still able to transmit power over the electromagnetic field.
As an example, 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.