Yeast are microorganisms. They are single-celled fungi. There are about 1,500 different species of yeast. Most reproduce asexually, by budding. Some use binary fission to reproduce asexually.
A particular species of yeast, Saccharomyces cerevisiae, has been used for a very long time. It is used for baking bread. Another use is as an additive to help the fermentation of alcoholic drinks, such as beer. It is also considered to be a model organism for modern cell biology. Other yeasts can cause infections in humans (they are pathogens). Yeast has also been used to make electricity, or to make ethanol (as a biological fuel for cars).
Dry yeast (usually, yeast is sold like this)