This is an important topic, all good technicians not only need to know what a transformer is and what they look like but also need to understand the theory behind how they work. It is also good to understand the different types of construction and there applications. There is a lot more to transformers than you may realize.
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors — the transformer's coils. A varying current in the first or primary winding creates a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the "secondary" winding. This effect is called mutual induction.
If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (VS) is in proportion to the primary voltage (VP), and is given by the ratio of the number of turns in the secondary (NS) to the number of turns in the primary (NP). To read more click here and here.