How is yeast is being used in the baking of bread?

I'm not doing your homework for you, but yeast basically eats sugars which are present in the dough. At the correct temperature and moisture levels, yeast replicates in the dough and its waste products (alcohol and CO2) make the dough rise. After a certain point, the yeast exhausts itself (either no more sugar or it 'chokes' on it's own waste products) and you have leavened dough for baking bread.