Yeast is a single-celled fungus with carbon dioxide and alcohol. The carbon dioxide is useful in baking for leavening while the alcohol adds flavor to baked goods and is useful in beer brewing and winemaking.
A natural yeast starter is a cultivated form of yeast. Grain, typically wheat flour or rye flour, is mixed with water and left out to naturally ferment and cultivate a thriving community of natural yeasts. This is used to create naturally leavened bread.