A tart differs from a pie in several ways: a tart is baked in a straight-sided pan which may or may not be fluted; a pie is baked in a sloping-sided pan. A tart is usually half the depth of a pie and is freestanding—removed from the pan and placed on a platter for serving. A tart may have a top crust (lattice or whole), though it is not usual.
You may use many of the same fillings in tarts as in pies, but I consider tarts more refined and less rustic and homey than pies. Fruit is featured in many of these tarts; a few call for rich fillings, which are more practical in a low tart shell than in a deep pie form. A marbled filling of dark and white chocolate sounds appealing as a tart, but too rich as a pie.