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. It is also used in fermentation to make ethanol. This method makes many drinks, such as beer or wine. Ethanol is also used as fuel and as feedstock to make other organic chemicals. Yeast can be used to make electricity.
Yeast is also a model organism for studying cell biology. Some yeasts can cause infections in humans (they are pathogens).
Dry yeast (usually, yeast is sold like this)