A pie is a baked dish which is usually made of a pastry dough 
Oh, I like pie!  Apple pie especially.
They have casing that covers or completely contains a filling of various sweet or savoury ingredients.
I prefer sweet pies to savory pies.  
funny enough,  Pies are defined by their crusts
Which is the best part, especially if it's flaky!
I always go for the filling, i'm australian so I love my traditional lamb pie, A "top-crust" pie has the filling in the bottom of the dish and is covered with a pastry