Yeasts are eukaryotic single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and 1,500 species are currently identified. They are estimated to constitute 1% of all described fungal species. Yeasts are unicellular organisms that evolved from multicellular ancestors, with some species having the ability to develop multicellular characteristics by forming strings of connected budding cells known as pseudohyphae or false hyphae. (extracted from Wikipedia)

In breadmaking, the most common used type of yeast is Saccharomyces cerevisiae, also known as baker's yeast or commercial yeast, also used in the beermaking process.