Electronics Forum / Basics / November 2008

A transformer is a device that transfers electrical energy from one
circuit to another through inductively coupled electrical conductors.
A changing current in the first circuit (the primary) creates a
changing magnetic field; in turn, this magnetic field induces a
changing voltage in the second circuit (the secondary); this is called
mutual induction. By adding a load to the secondary circuit, one can
make current flow in the transformer, thus transferring energy from
one circuit to the other.

http://electronicstopics.blogspot.com/2008/10/transformer.html