A pie is some food surrounded by pastry (pastry is a mixture of flour and butter). It is normally baked in the oven. The content of the pie can be sweet or savoury. Typical examples of pies are "steak and kidney pie" or "apple pie". There are variations of the pie such as cornish pasties. Pasties were originally invented so that working men could take their food to work with them. Someone told me that miners in the Cornish tin mines invented the cornish pastie. I suppose that if you work in a mine it is too much trouble to come to the surface to have lunch.