Yeast is a Biological Leavening Agent, which is a microscopic organism that, when fermented, releases carbon dioxide into a flour mixture. For yeast to be properly fermented it needs moisture, sugar and carbohydrates. Fermentation is a biological reaction that splits complex components into simpler substances.
The science behind Yeast is relatively straightforward. Yeast works to convert the sugar into ethyl alcohol(eventually evaporates) and carbon dioxide, and the carbon dioxide allows the dough mixture to eventually rise. The fermentation stage is also referred to as the “Proofing” stage. Proofing is a slow process and gives the yeast time to reproduce and ingest carbohydrates in the flour. Warm temperature helps to speed up this process but it will still be a relatively slow process. When working with yeast, it is best to use warm water in addition to leaving it in a very warm place to rise. Out of all leavening agents, yeast delivers the leading increase in size. Fermented yeast also excretes a certain flavor into the baked product creating a unique flavor. It is best to use a biological agent such as yeast when working with mixtures that can hold in the carbon monoxide gas the fermented yeast creates. Such mixtures would include bread, focaccia, buns, rolls, etc. Read more about different types of Leavening Agents here