CoCo asked about bread starters,
I'm thinking of making my own - does it act like yeast in the dough?
Yes, just so. In the air around us are billions of spores of microscopic wild yeast and bacteria. given a hospitable environment, these like flowers of the plant kingdom will go to work for you, merrily converting the carbohydrates in your dough to simpler forms and releasing bubbles of CO2 in the process. It's these bubbles of CO2 that leaven your loaf, making it rise. When the dough goes into the oven, the higher temperatures cause the little bubbles to expand even more and also kill the yeasts and bacteria.
It's pretty easy to "capture" a starter, but sometimes things don't go well or the organism that grows is not the right sort. Simply dump out the starter and begin again. Varying the wetness of your starter can influence which kind of yeast or bacteria thrive there, as can varying the pH of the mix by using a bit of fermented milk (buttermilk, yogurt, sour cream, etc) or acid fruit juice (just a bit - pineapple juice works well) in the starter mix.
...OUTLAW