Cream Pies: Where eggs are used with heavy cream to make a silky, thick base. These are a subset of custard pies, and the boundaries between them often blur. If you're going to throw a pie at someone, cream pies are the obvious choice. I
Custard Pies: Any pie where eggs are used to set a liquid. Pumpkin pie's a good example. I would suggest that lemon and lime pies fall into this category as well, along with pudding pies and most chocolate pies. Cakespy Note: another one which has fascinated us in the past is the Hoosier Pie, a kind of sugar-custard pie which seems to be big in the American mid-west. Photo left: Pumpkin pie from the North Hill Bakery, Seattle.
Mousse Pies and Chiffon Pies: Egg whites are the major player here, though many recipes call for gelatin as well.
Nut Pies: I lump all pies requiring nuts set with corn syrup in this category (including peanut pie, though peanuts are legumes, not nuts); Walnut pie, Kentucky Derby pie, Pecan pie . . . while some of the recipes contain eggs, the eggs don't set a liquid, which is what I think separates nut pies from custard and cream pies. I might lump in sweet bean-based pies, too, since the beans were used when people didn't have nuts.
But now, to answer the pressing pie questions:
What is the difference between a tart and a pie? No, tarts aren't just pretentious pies. A tart is always uncovered, and generally made in special, delicately shaped tins. So by this logic a tart is a pie, while a pie is not necessarily a tart. However, the general connotation is that a pie is more rustic, peasant fare, where a tart is more refined. Also, pastry chef Chris Jarchow (who, incidentally, made the tart pictured to the left) points out that tarts are generally defined further by the use of Pâte Sucrée (sweetened crust) as opposed to Pâte Brisée (unsweetened crust), which is what you'd see on say, an apple pie.