Complex sentence: Generally, there are two pre-ferment varieties: sponges based upon baker's yeast and the starters of sourdough (wild yeasts and lactic-acid bacteria); however, some bakers use the term to refer only to the yeast variety.
Split into simpler sentences: Generally, there are two pre-ferment varieties: sponges, based upon baker's yeast, and the starters of sourdough, based on wild yeasts and lactic-acid bacteria. Some bakers, however, use the term to refer only to the yeast variety.