In Japanese, ema literally means "horse picture."  The name originates from the time when real horses were  offered by the wealthy in exchange for blessings at shrines.  At some of the more central and popular shrines, such as the Meiji shrine in Tokyo, one can find ema written by tourists in many different languages!

Ema are wooden plaques on which Shinto worshippers write their prayers or wishes.  The ema are then left hanging up at the shrine so that the gods (kami) can read them.  They may have pictures of animals or other Shinto images painted on them, and they often have the word negai, or wish, written along the side.