It seems that there is a difference between pies and tarts in AmE and SAfE.
In South African English the primary difference is that a pie has a pastry crust on top, whereas a tart doesn't.
It seems that in American English the difference is mainly concerned with size.
That's the biggest part of the difference in AmE as I know it, a pie being larger (generally over 8" in diameter) and served by the slice and a tart being smaller (generally under about 4" in diameter) and served one to a person rather than being cut into slices. If the diameter is under about 2" it's often called a "tartlet".
However there are some recipes that are called tarts, no matter how large the pan used is. In these cases, (like tarte tatin and Bakewell tart) the name has been imported with the recipe.
The other difference, according to the cookbooks (which may or may not be reflected in common usage) is the type of pan that it's baked in. A pie-pan has smooth sides; the sides of a tart pan are fluted, producing a crust that has a scalloped outside profile, rather than a smooth circle.
Then there are meat pies, which can be 2" or so, and up.