I mentioned already once or twice that I like to at potatoes into bread dough. In combination with a preferment they make the crumb soft and fluffy.
A classic preferment consist of flour, water, sometimes salt and microorganisms. In a sourdough the microorganisms are different lactobacteria and yeasts, while in a biga, poolish or pâte fermentée you will find bakers yeast (because you added them). A preferment will ferment for some time (mostly 12-16 hours) and in this time the microorganisms will release byproducts of their metabolism into the preferment. That makes the flavour of the dough (and later of the bread) complex and deep.