Yeast is a single-celled microorganism of the fungus family. The species used in cooking, Saccharomyces cerevisiae (different strains used for different purposes), converts carbohydrates into carbon dioxide (used in baking as a leavening agent) and alcohol (used in alcoholic beverage production) through fermentation.  You can use fresh yeast, dried active yeast or fast action (instant) yeast. Fresh yeast needs to be ‘creamed’ in liquid (dissolved until creamy and bubbling) and you need roughly 30g per 500g loaf of white bread. Dried active yeast needs to be ‘activated’ in water and you need roughly 15g per 500g loaf of white bread. Fast action or instant yeast can be added straight into your flour with no further action required and you need roughly 7g per 500g loaf of white bread.