A transformer is a simple electrical machine most commonly used to change voltage levels from one value to another in different parts of electrical circuits.
A transformer can step voltage up or down. A basic transformer has two tightly coiled loops of wire called 'windings' coupled together, a primary winding and a secondary winding. These are usually wound around a laminated soft iron core. When a current is introduced into the primary winding, a voltage is induced in the secondary winding. Varying the relative number of winding turns in the two windings determines the variation between the input and output voltages.
A step up transformer increases the voltage from the primary winding to the secondary winding, while a step down transformer decreases the voltage from the primary winding to the secondary winding.
A common application of transformer action is the vehicle ignition coil. The ignition coil steps up the relatively low primary winding input voltage to a secondary high tension output voltage.