Yeast has been one of those great technical advances in the sake world -- one factor that separates great ginjo of today from the run-of-the-mill sake of yesteryear. Over the last 10 years or so, dozens of new yeast strains have been developed and incorporated into sake brewing.
Chemically, yeast converts sugar to alcohol and carbon dioxide. It is the heart of the creation of all alcoholic beverages. But different yeast strains will produce different things, like esters, alcohols, acids and other chemical compounds that affect the nuances of fragrance and flavor.
Each yeast will give rise to its array of chemical compounds, with scary names like ethyl n-caproate and isamyl acetate. Which esters, alcohols and other compounds are produced is highly dependent on the temperature at which fermentation takes place.