Fermentation is the process by which microorganisms convert sugar to acids or alcohol in the absence of oxygen.
Yogurt and curd are made using fermentation of milk by bacteria such as Streptococcus and Lactobacillus species. Alcoholic beverages like beer, wine, brandy, whisky etc are made using fermentation of grains, grapes, rice etc by yeast. Bread is also made by fermenting dough using yeast. Gasohol is a fuel made using gasoline and alcochol which involves fermentaion too. Apart from these, fermentation is also used in the preparation of certain medicines.