Electrical transformers are devices that transfer electrical current from one circuit to another through electrical induction. They consist of two coils of wire, wrapped around an electrically conductive core, which is usually made from laminated iron.
A basic 'step down' transformer
The coil on the left of the diagram is called the "Primary winding", carries the input current. The coil on the left is known as the "Secondary winding", and carries the output current. (more...)