A pie is a baked dessert with a pastry crust. If your favorite kind of pie is apple, you might order it with a scoop of vanilla ice cream on top.
Most pie is sweet, but there are also meat and vegetable pies, often called "pot pies." Like dessert pies, these savory dishes are often topped with pastry. Other pies have a bottom crust and an open top, while some have both a top and bottom pastry crust. If something is described as "easy as pie," it's very easy — though it's arguable whether it's really all that easy to make a pie.