Yeasts are a polyphyletic group of unicellular fungi. Usually the term yeast is used synonymously with baker's yeast - Saccharomyces cerevisiae.
Yeasts include organisms from the phyla Ascomycota and Basidiomycota and is therefore a polyphyletic group. In common practice, yeast or budding yeast refers to Saccharomyces cerevisiae. Fission yeast refers to Schizosaccharomyces pombe.
Saccharomyces is a commercially important organism, used in fermentative production of alcoholic beverages and breads. It is also one of the simplest eukaryotes and is commonly used as a model organism to study basic eukaryotic cell-cycle and gene regulation pathways.