Pizza is an open-faced sandwich. Like many sandwiches, pizza has a bread base and is filled with vegetables, cheese, meats, mushrooms and other foods on the top, much like sandwiches. It only lacks the top layer of bread to be considered a true sandwich.
However, some people, especially Italians, use the terms pizza and pie interchangeably. This is because pizza is cut into small cake-like portions before serving. Also, pizza means pie in Italy.