The pentacle is a five-pointed star, or pentagram, contained within a circle. The five points of the star represent the four classical elements, along with a fifth element, which is typically either Spirit or Self, depending on your tradition. The pentacle is probably the best-known symbol of Wicca today, and is often used in jewelry and other designs. Typically, a pentacle is traced in the air during Wiccan rituals, and in some traditions it is used as a designation of degree. It is also considered a symbol of protection, and is used in warding in some Pagan traditions.
There is a theory that the pentacle originated as the symbol of a Greek agricultural and fertility goddess named Kore, also called Ceres. Her sacred fruit was the apple, and when you cut an apple in half crossways, you find a five-pointed star! Some cultures refer to the apple-star as the "Star of Wisdom", and so apples are associated with knowledge.
A pentacle has magical properties associated with the element of Earth, but it contains aspects of all the other elements as well. In June 2007, thanks to the efforts of many dedicated activists, the United State Veteran's Association approved the use of the pentacle for display on the headstones of Wiccan and Pagan soldiers killed in action.
Although it's not something used in all Pagan traditions, some magical systems connect different colors to the points of the pentacle. As part of that, the colors are often associated with the four cardinal elements -- earth, air, fire and water -- as well as spirit, which is sometimes considered the "fifth element."