The most common ingredient used to make traditional bread rise is yeast. In particular, baker's yeast—usually the species Saccharomyces cerevisiae—feeds on fermentable sugars in dough and releases carbon dioxide during fermentation.
That carbon dioxide becomes trapped in the stretchy gluten network formed from flour and water. The gas bubbles expand as the dough rests and heats in the oven, producing bread that is lighter and softer than an unleavened loaf. Yeast also creates ethanol and flavor compounds, although most of the alcohol evaporates during baking.
Yeast is not the only way to raise bread. Baking soda and baking powder chemically release carbon dioxide and are common in quick breads, biscuits, muffins, and soda bread. Sourdough uses wild yeasts together with acid-producing bacteria, while some breads rise through steam or mechanically incorporated air.
A common mix-up is confusing yeast with gluten. Yeast produces much of the gas, while gluten helps hold that gas in the dough. Without enough structure, fermentation can occur but the loaf will not retain a high, airy shape.