A Ouija board is a device used to communicate with alleged supernatural powers, usually (but not always) ghosts. It consists of a board with all of the letters of the alphabet, along with the digits 0–9 and short words like "yes", "no" and "bye". It also has a pointer of some sort, either a triangle or a magnifying glass, that passes easily over the board. According to believers, once everyone has a hand on the pointer, then the spirits will move it over the letters to answer questions asked by the group. In actuality, it is a classic example of the ideomotor effect. (Of course, fundamentalist Christians who believe in them think that demons are the ones controlling the pointer.) The idea that one person might be unconsciously moving the device is usually not considered.
Ouija boards are almost exclusively used by teenagers seeking cheap thrills, yet some fundamentalist Christians see them as a major route to demonic possession. They also associate the boards with the New Age movement and neopaganism, even though neither of the groups use them. The main organised religion to use the boards is the Spiritualist church, who seem to be ignored by most people.
The name comes from "oui", which means "yes" in French, "ja", which means "yes" in Dutch and German, and "board", which means "board" in English (because it's a board).