An apple pie is typically made with a double crust (pie dough on the bottom and top of the filling), while an apple tart has a single crust (just on the bottom).  Apple tarts also tend to have a higher ratio of fruit to dough, making them slightly lighter than pies. And because there's no top crust, the filling is more exposed, which allows it to caramelize and get nice and bubbly in the oven.