Transformer

There may hundreds of billion of transformers in the world, you can count how many transformer around you, usually there is more than one transformer in any of the electrical appliances (except handheld products like pda & mp3 where the transformer is located in a power plug form), from this you can imagine the number of transformer in the world.
For ideal transformer, i.e. no loss,
input power = output power
input voltage * input current = output voltage * output current
For typical transformer, i.e. with loss (iron loss & copper loss)
input power > output power
input voltage * input current > output voltage * output current
the excess power will be transformed to heat.

圖片參考：http://content.answers.com/main/content/img/ahd4/A4trsfmr.jpg

Transformer is electrical device used to transfer an alternating current or voltage from one electric circuit to another by means of electromagnetic induction. The simplest type of transformer consists of two coils of wire, electrically insulated from one another and arranged so that a change in the current in one coil (the primary) will produce a change in voltage in the other (the secondary). In many transformers the coils are wound on a core made of a material with high magnetic permeability; this intensifies the magnetic field induced by the current in the primary, increasing the transformer's efficiency. Neglecting power losses (which are made small by careful design), the ratio of primary voltage to secondary voltage is the same as the ratio of the number of turns in the primary coil to the number of turns in the secondary coil. The primary and secondary currents are in inverse proportion to the number of turns in the coils. The primary and secondary impedances are in the same ratio as the squares of the numbers of turns in the primary and secondary coils. For example, if a 10-volt, 2-ampere alternating current were to flow through a 10-turn primary of a transformer, theoretically a 20-turn secondary would exhibit a 20-volt, 1-ampere alternating current, with the output impedance four times as great as the input impedance. Transformers are frequently classified according to their uses; the details of construction depend on the intended application. Power transformers are generally used to transmit power at a constant frequency. Audio transformers are designed to operate over a wide range of frequencies with a nearly flat response, i.e., a nearly constant ratio of input to output voltage. Radio frequency (RF) transformers are designed to operate efficiently within a narrow range of high frequencies.
I hope this can help with your understanding. =)