Bread is a staple food prepared from a dough of flour and water, usually by baking. Throughout recorded history it has been popular around the world and is one of the oldest artificial foods, having been of importance since the dawn of agriculture. 
There are many combinations and proportions of types of flour and other ingredients, and also of different traditional recipes and modes of preparation of bread. As a result, there are wide varieties of types, shapes, sizes, and textures of breads in various regions. Bread may be leavened by many different processes ranging from the use of naturally occurring microbes (for example in sourdough recipes) to high-pressure artificial aeration methods during preparation or baking. However, some products are left unleavened, either for preference, or for traditional or religious reasons. Many non-cereal ingredients may be included, ranging from fruits and nuts to various fats. Commercial bread in particular, commonly contains additives, some of them non-nutritional, to improve flavor, texture, color, shelf life, or ease of manufacturing.
Depending on local custom and convenience, bread may be served in various forms at any meal of the day. It also is eaten as a snack, or used as an ingredient in other culinary preparations, such as fried items coated in crumbs to prevent sticking, or the bland main component of a bread pudding, or stuffings designed to fill cavities or retain juices that otherwise might drip away.
Partly because of its importance as a basic foodstuff, bread has a social and emotional significance beyond its importance in nutrition; it plays essential roles in religious rituals and secular culture. Its prominence in daily life is reflected in language, where it appears in proverbs, colloquial expressions ("He stole the bread from my mouth"), in prayer ("Give us this day our daily bread") and even in the etymology of words, such as "companion" and "company" (literally those who eat/share bread with you).
Yeast is an ingredient in bread. It produces bubbles of gas which cause the dough to rise.