Sourdough or sourdough bread is a bread made by the fermentation of dough using wild lactobacillaceae and yeast. Lactic acid from fermentation imparts a sour taste and improves keeping qualities.
When wheat flour comes into contact with water, the naturally occurring enzyme amylase breaks down the starch into the sugars glucose and maltose, which sourdough's natural yeast can metabolize. With sufficient time, temperature, and refreshments with new or fresh dough, the mixture develops a stable culture.
Sourdough is a stable culture of lactic acid bacteria and yeast in a mixture of flour and water. Broadly speaking, the yeast produces gas (carbon dioxide) which leavens the dough, and the lactic acid bacteria produce lactic acid, which contributes flavor in the form of sourness. The lactic acid bacteria metabolize sugars that the yeast cannot, while the yeast metabolizes the by-products of lactic acid fermentation.
And, then, there is this fascinating interview with a researcher, talking about the yeasts and bacteria in starters: