The words tart and galette identify similar types of sweet or savory pastry, yet there is a slight distinction between the two. A tart consists of a shallow, straight-sided pastry that is filled before or after baking and has no top crust. Typically, tarts are formed in tart pans (many with the characteristic scalloped edge), but they may also be shaped directly on a baking sheet, often with the help of a frame or ring. Tarts may be round, rectangular, square, or miniature (also called a tartlet). While older cookbooks define tart as the French term for pie, today we make a distinction between the two, using pie to refer to a deeper-dish pastry with sloped sides and often, but not always, a top crust.
The term galette has a looser definition that is tied to the French word galet, meaning a smooth, flat pebble. Some sources explain that the very earliest breads were indeed galettes—simple, unleavened breads made by smearing thick cereal paste on hot stones. By-definition any flat, round pastry or cake-like creation constitutes a galette. This includes round, shortbread-like butter cookies, the thin buckwheat crêpes of Brittany, and something as simple as potato cakes.
Today, however, the term galette is used primarily to refer to rather rustic, free-form tarts-made with a single crust of pastry or bread dough, like a pizza. If the filling is very moist, the sides of a galette may be folded up and over to contain the juices. Alternatively, the edges may be simply crimped and left rather flat. Italian cooks use the term crostata in place of galette.