A shop selling baguettes may also have croissants, éclairs, sandwiches, and cakes in the window. That does not make all of them boulangerie. French distinguishes between boulangerie, pâtisserie, and viennoiserie, even when you find all three in the same place.
Boulangerie begins with bread
A boulangerie is a bakery centered on making and selling bread. The word also refers to the trade of bread-making.
A boulangerie may sell much more than bread. Croissants, sandwiches, quiches, and desserts are common additions, so what you find inside does not always tell you which category a food belongs to.
The people who make the bread are un boulanger and une boulangère.
For the names of common breads and baked goods, see Bread and baked goods.
Pâtisserie covers pastry-making and pastry shops
Pâtisserie has a broader role than the English word "pastry." It can refer to the craft of making cakes and pastries, the products themselves, or a shop specializing in them.
Éclairs, macarons, mille-feuilles, and tarts are examples of pâtisseries. The person who makes them professionally is un pâtissier or une pâtissière.
You will also see boulangerie-pâtisserie on storefronts. The two terms together indicate that the business works with both bread and pastry rather than fitting neatly into only one category.
For the vocabulary used for cakes and other sweets, see Desserts and sweets.
Viennoiserie is not another kind of shop
Viennoiserie is where the distinction becomes important. It refers to a category of baked goods, not an equivalent third type of shop.
Croissants, pains au chocolat, pains aux raisins, and brioches are des viennoiseries. You will commonly find them in a boulangerie, even though they are not bread.
The term comes from Vienne, the French name for Vienna, reflecting the Austrian influence on the development of this tradition of baking in France.
The boundaries overlap
These distinctions describe crafts and categories, not three completely separate places to buy food. A boulangerie can sell viennoiseries and pâtisseries. A business may identify itself as a boulangerie-pâtisserie because it does both.
That overlap explains why translating all three simply as "bakery" or "pastry shop" can hide useful information. The French terms tell you something about the type of baking or product involved, even when everything is sold under one roof.
From learning to practice
When you see boulangerie, pâtisserie, or viennoiserie, think about what the word describes rather than looking for a single English translation. That distinction will help you make sense of shop signs, menus, and displays.