Yeast is used in bread making to make the dough rise from a dense mass of flour to airy bread. Yeast causes the process of fermentation to happen in bread, which is crucial to the bread rising.
Fermentation is the process by which extra sugars are consumed while carbon dioxide and alcohol are emitted as byproducts from the consumption. The way that carbon dioxide is released from the sugars that are created by yeast is similar to the way that a bubble is blown with a piece of bubblegum. The carbon dioxide infiltrates the small air pockets that are present in the dough and begins to expand them to a level that makes the bread rise.
Gluten is an important product found in bread that makes the bread stay together without crumbling. It works as a type of glue and is present in a network in most types of bread. The addition of yeast to bread dough allows the gluten to become strengthened so that it can increase the network that holds the bread together. Gluten can also be strengthened by kneading the bread dough together. This is evident in the physical appearance of dough. Dough that has not been kneaded looks significantly different from dough that has been.