Transformers operate by the principles of electromagnetic induction, transferring energy from one circuit to another.  Transformers are typically constructed with 2 or more windings wound around a ferromagnetic “core”.  A changing voltage (AC or pulsed DC) applied across the input winding results in a varying primary current, generating a changing magnetic field concentrated within the “core”.  This varying field induces an electric current in the “secondary” or “output” winding(s) via electromotive force – Faraday’s Law of Electromagnetic Induction.  The output voltage is stepped up or down from the input voltage, proportional to the “turns ratio”, without changing the operating frequency.