Bread is nothing but a a pile of flour and water without its parent, yeast. Yeast is a living, single-celled organism classified as a fungi and reproduces asexually. Yum. In order for the yeast to grow it needs to be placed in the prefect moisture and temperature and also needs the properly food supply. Its growth, or reproduction, occurs by budding, where swelling forms on the surface of each yeast particle. Unlike other creatures, reproduction for yeast is short, taking around only 20 minutes.
There are multiple types of yeast but mainly two for common cook, compressed and dry. Dry yeast has 90% of its moisture removed and is able to live 6 months outside of the refrigerator. Compressed yeast has only 30% of its moisture removed and needs to be refrigerated. Both yeast come dormant and become active when mixed with a stimulate such as hot water or milk.
To make bread rise, the yeast eats sugar and passes gas. Well, more like releases carbon dioxide making the dough lift up. Rising also occurs when the bread is in the oven. The heat causes the yeast to go crazy and release more carbon dioxide quickly.
Yeast doesn't only make the bread rise, it also strengthens and adds flavor. Flour and water alone produce gluten which causes the dough to become more elastic and smooth, but yeast greatly increases the amount produced if mixed in the combination. Also, yeast causes fermentation which adds flavor to bread and other components like amino acids and acetic acid.
Just a word of advice, sometimes yeast dies before you can use it in cooking. To test if it is alive, put some yeast in water. The yeast should begin budding and you shouldn't have to waste any flour.
Want to know more?
http://science.howstuffworks.com/life/fungi/yeast-info.htm
http://www.wisegeek.org/why-does-yeast-make-bread-rise.htm
http://www.finecooking.com/articles/yeast-role-bread-baking.aspx?pg=2
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