From Armenian LAVASH, to British CRUMPETS, to Canadian LORNE COTTAGE ROLLS and POP OVERS, to French CROISSANTS and BRIOCHES, to Italian PANETTONE, to IRISH SODA BREAD, bread comes in thousands of forms.
What do they have in common? On the most basic level, they all involve cooking a mixture of milled grains and water.
Some are amazingly simple: Matzoh, for example, is nothing more than flour and water, baked until crisp. Raised breads, on the other hand, involve the complex interactions between flour and the leaveners that give them their porous, tender quality.
Leaveners come in two main forms: baking powder or soda and yeast.
Baking powder or baking soda work quickly, relying on chemical reactions between acidic and alkaline compounds to produce the carbon dioxide necessary to inflate dough or batter (more on this later).
Baking powder and baking soda are used to leaven baked goods that have a delicate structure, ones that rise quickly as carbon dioxide is produced, such as quick breads like cornbread and biscuits.
Yeast, on the other hand, is a live, single-celled fungus. There are about 160 species of yeast, and many of them live all around us. However, most people are familiar with yeast in its mass-produced form: the beige granules that come in little paper packets. This organism lies dormant until it comes into contact with warm water. Once reactivated, yeast begins feeding on the sugars in flour, and releases the carbon dioxide that makes bread rise (although at a much slower rate than baking powder or soda). Yeast also adds many of the distinctive flavors and aromas we associate with bread.
But leavening agents would just be bubbling brews without something to contain them. Here's where flour comes in. There are lots of different types of flour used in bread, but the most commonly used in raised bread is wheat flour. This is because wheat flour contains two proteins, glutenin and gliadin, which, when combined with water, form gluten. As you knead the dough, the gluten becomes more and more stretchy. This gum-like substance fills with thousands of gas bubbles as the yeast goes to work during rising.
For more detailed information on the science of bread go to http://www.exploratorium.edu/cooking/bread/bread_science.html