Yeast is a kind of fungi.
Yeasts are fungi that grow as single cells, producing daughter cells either by budding (the budding yeasts) or by binary fission (the fission yeasts). They differ from most fungi, which grow as thread-like hyphae. But this distinction is not a fundamental one, because some fungi can alternate between a yeast phase and a hyphal phase, depending on environmental conditions. Such fungi are termed dimorphic (with two shapes) and they include several that cause disease of humans.
Here are several examples of yeasts and dimorphic fungi:
the common baker's yeast, Saccharomyces cerevisiae
the genus Cryptococcus, which includes C. neoformans, a pathogen of humans
the dimorphic fungus Candida albicans which can be a significant pathogen of humans
some of the common leaf surface yeasts.
Yeasts grow typically in moist environments where there is a plentiful supply of simple, soluble nutrients such as sugars and amino acids. For this reason they are common on leaf and fruit surfaces, on roots and in various types of food. With few exceptions, they are unable to degrade polymers, such as starch and cellulose which are used by many hyphal fungi.
To learn more about yeast and fungi, visit Biological Sciences - The University of Edinburgh.
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