What is a starter / levain / sourdough?

Sourdough (or more formally 'levain') refers to the process of leavening bread by capturing wild yeasts in the dough. Sourdough's have a distinctively tangy or sour taste (hence its name), due mainly to the lactic acid and acetic acid produced by the yeast. (1/2)

Levains are made using a small amount of "starter" dough (sometimes known as "the mother"), which contains the yeast culture, along with flour, water and salt. Part of this resulting dough is then saved to use as the starter for the next batch. It is not uncommon for a baker's starter dough to have years of history.