A transformer basically is a very simple static (or stationary) electromagnetic device that works on the principle of Faraday’s law of induction by converting electrical energy from one value to another.
A transformer does this by linking together two or more electrical circuits using a common oscillating magnetic circuit which is produced by the transformer’s core. A transformer operates on the principle of electromagnetic induction in the form of mutual induction.
Mutual induction is the process by which a coil of wire magnetically induces a voltage into another coil located in close proximity to it. Therefore, we can say that transformers work in the “magnetic domain” and transformers get their name from the fact that they “transform” one voltage or current level into another.