"Ema" is a wooden board with a picture of a horse, which is dedicated to a temple or a shrine when making a wish, or when showing gratitude for the granted wish.
Ema sold at temples and shrines for personal dedication are small with a picture of a horse, and have space in the margin or on the back to write the wish and a person's name. Those dedicated by a large group of people are large, and are prepared by the dedicator, sometimes with pictures drawn by artists. It normally has a pentagon shape (shape of a house), which can be traced back to the fact that, in the past, they were made with a roof attached to a piece of the board.
According to the "Shoku-Nihongi" (the second in a series of chronicles about Japan) from the Nara period, a sacred horse for the god to ride was dedicated. However, horses were high-priced and could not be dedicated easily, and the temples and shrines receiving the horses had a hard time taking care of them. Therefore, those who could not dedicate a horse began substituting with statues of horses made from wood, paper, or clay, and from the Heian period, it was substituted with a picture of a horse drawn on a board.
Furthermore, in the Muromachi period, pictures other than horses were drawn. For example, in Inari-jinja Shrine, where a fox is the messenger, some have pictures of foxes. There are ema with the character 'め' (eye) and a mirror image of 'め' for the prevention of eye disease, and ema with the character '心' (heart) with a lock for the prevention of extramarital affairs. Practitioners of Japanese mathematics dedicated ema called sangaku, with the solution of the problem they solved written, while in Japanese martial arts such as kenjutsu (fencing) and jujutsu (classical Japanese martial art, usually referring to fighting without a weapon), ema with wooden swords such as halberd or sticks for bojutsu (art of using a stick as a weapon) attached to a list of pupils were dedicated. In the Azuchi-Momoyama period, ema by distinguished artists were lionized and emado (shrine building where ema were hanged) were built to exhibit them.
In the Edo period, praying for practical benefits such as the well-being of the family or prosperity of business became widespread among common people, and it is in the Edo period that people began dedicating small ema as we do today. Since the Showa period, students preparing for examinations often dedicate ema to pass exams at the Tenmangu Shrine where SUGAWARA no Michizane is honored as Tenjin (heavenly gods), the god of learning.
In recent years, many ema unrelated to horses, such as those of good-luck animals like albino Japanese rat snakes and festival scenery are made and valued highly as good-luck charm of shrines and temples, and are lionized as charms when visiting the shrines and temples. Additionally, fans watching the animation "Kamichu!" aired in 2005 and the animation "Lucky☆Star" aired in 2007, are drawing the characters on ema and dedicating them when they visit shrines where animations took place (the former at the Misode-tenmangu Shrine in Onomichi City, Hiroshima Prefecture, and the latter at the Washinomiya-jinja Shrine in Washimiya-cho, Saitama Prefecture).
Since around 2006, from a Private Information Protection Law perspective, some ema come with stickers to place over the wish, address, and names written on the ema.