A tart is not a pie although they are both baked pastries with fillings.
The key difference between a tart and a pie lies in the crust and also the presence or absence of a top crust.
Tarts often have a shortcrust pastry along with a shallow and open top.
And pies on the other hand often have a thicker and more robust crust and may also have a top crust.
Although not technically a pie, the tarts can be seen as a shallow pie as they are much more shallow in depth than regular pies.
Tarts are basically a baked dish that consists of a filling over a pastry base along with an open top that is nto covered in pastry like a pie would be.
The pastry for tarts are often a shortcrust pastry and the filling of the tart may be sweet or savory although modern tarts are often fruit based and sometimes with custard.
When it comes to what qualifies as a pie, the pies are defined by their crusts.
For example a filled pie (also single-crust or bottom-crust), has pastry lining the baking dish, and the filling is placed on top of the pastry but left open.
A top-crust pie also has the filling in the bottom of the dish and is covered with a pastry or other covering before baking.