From The Art and Popular Culture Encyclopedia
Pies are defined by their crusts. A filled pie (also single-crust or bottom-crust), has pastry lining the baking dish, and the filling is placed on top of the pastry, but left open. A top-crust pie, which may also be called a cobbler, has the filling in the bottom of the dish and the filling covered with a pastry or other covering before baking. A two-crust pie has the filling completely enclosed in the pastry shell. Flaky pastry is a typical kind of pastry used for pie crusts, but many things can be used, including baking powder biscuits, mashed potatoes, and crumbs.
Pies can be a variety of sizes, ranging from bite-size to ones designed for multiple servings.