Transformers have the effect of increasing or decreasing the voltage in a circuit. We may want to increase it – using a step up transformer  because for example, electricity transmission over long distances at high voltage over the National Grid means lower energy losses than electricity transmission at lower voltages. We would transform down to a lower and safer voltage – using a step down transformer  to supply it into people's houses. A typical transformer consists of loops of wire on an iron core as shown.
<Transformers only work on ac circuits ie when the input voltage is an alternating voltage. The process is this:

An alternating current in the primary circuit on the left induces an changing magnetic field in the transformer core according to Faraday's Law and Lenz's Law.

This changing magnetic field in the core induces a current in the secondary coil, again according to Faraday's and Lenz's Laws.
For a perfect transformer, such that there are no energy losses, so that the efficiency is 100%, we may use the equations,
and
In fact transformers are not 100% efficient. We may reduce energy losses by
Laminating the transformer core. The changing magnetic field induces eddy currents in the core itself. Since the core has resistance, energy losses will result here. If we laminate the fore reduces the eddy currents.
The wires in the primary and secondary coils have resistance. We could use thicker wires. Maybe superconducting wires may appear in the next few years.
Each time the core is magnetized and demagnetized, hysteresis losses result. We reduce these by using a soft iron core, which is easily magnetized and demagnetized.