Nearly every tradition of Wicca (and many Pagan paths, as well) uses the pentacle. Not to be confused with the pentagram (a five-pointed star), the pentacle is a flat piece of wood, metal, clay, or wax inscribed with magical symbols. The most commonly seen symbol, however, is the pentagram itself, which is why the two terms are often confused.
In ceremonial magic, the pentacle is used as a protective talisman. However, in most Wiccan traditions it is seen as representative of the element of Earth, and can be used on the altar as a place to hold items that are going to be ritually consecrated. You can make your own, or buy one commercially. The one in the photo was made with a wood-burning kit and a piece of sanded pine purchased from a craft store.