It reacts with the sugars to make it rise.
Yeast doughs are used to make countless types of bread. "Dishes" made with yeast dough would be pizza and calzone.
Baguette dough is a lean dough.
Leavened bread has "leavening" added to the dough. Leavening can be yeast, baking soda or baking powder, or natural yeasts absorbed from the air which create sour dough. Unleavened bread does not have leavening of any kind.
Without flour (usually wheat) there would be no bread! Bread is made from flour. What makes a difference is the addition of yeast (balm) and warmth that causes the yeast to multiply and makes the dough rise and become lighter.
Bakers yeast ,a commercial preparation made from dried cells of the fungus namedSaccharomyces cerevisiae, it is used as a leavening agent in doughs during baking.
If you don't include water (or a similar liquid) in the ingredients, then it won't BE bread. Bread is made from dough, and water is a vital ingredient in dough. Without water, "breadmaking" would consist of heating a mixture of dry flour and dry yeast and dry salt, and all you'd end up with would be a mixture of burnt dry flour, burnt dry yeast, and burnt dry salt.
To allow the dough to rise so you get a lighter, less dense end product.
Recipes will vary depending on the type of pastry or bread your making but the are the basic ingredients, Short crust pastry is Flour, Fat a little salt and water. Bread dough is Flour, Fat, Yeast, a little sugar (to activate the yeast) salt and water.
Yeast is important in bread making as yeast is the reactant with heat in the oven that makes it rise. It also releases Carbon Dioxide (CO2) which gives you the tiny holes in the bread. The yeast cells in bread dough ferment sugars and produce gas (carbon dioxide). This will make the dough rise. If there is not enough yeast, the dough will not rise sufficiently and become less fluffy after baking. Without yeast you get unleavened bread. With too much yeast the dough becomes very light, and in the oven will collapse. in both cases the bread is still completely safe to eat. The amount of yeast per bread is very much a matter of taste and varies from country to country.
Yeast is a leaven. A chemical reaction between the yeast and water creates tiny gas bubbles, making dough expand.
Neither is great, but of the two, whole wheat would be better. It is a little harder to breakdown into sugar so you will not spike quite as much or as quickly. Actually I've hear quite the contrary. Sourdough bread is sour due to lactic acid released by the bacteria fermenting in the dough. The lactic acid is what keeps the blood sugar from rising.
Adding salt to bread dough controls the action of the yeast and improves the flavor. Bread made without salt will have a coarser texture and a blander flavor than bread made with salt.