Definition of Sourdough
Sourdough is wild yeast and bacteria fermented without the addition of commercial yeast.
How to Make:
Organisms necessary for sourdough are generally present naturally on the surface of grain and flour. Sourdough can be made by making a paste or dough of flour and water. The mixture can be left in a cup or other food vessell, either covered or partially covered (to prevent drying out). Fermentation can occur within several days, though often this first activity does not contain long term sustainable yeast and bacteria.
Sometimes it takes several weeks to develop fermentation that is sustainable and contains suitable yeast and bacteria to successfully rise bread dough.
Please see this blog for a good technique to make sourdough: http://sourdough.com/blog/sourdom/beginners-blog-starter-scratch
Agricultural chemicals can reduce the quality and quantity of yeast and bacteria. Organic flour is generally preferred to make and maintain sourdough.
Rye flour in particular is known for good fermentation properties. Many bakers add rye flour to their wheat based sourdough, particularly when attempting to 'capture' sourdough for the first time.
'Starter' is a general term used to describe sourdough before it is added to bread dough. Starter needs to be 'fed' with flour or other suitable foods (eg. sprouts, soaked grain) to keep it active and make it strong enough to raise bread dough.
Starter can be fed twice a day (12 hour cycles) at warm room temperature, or once a day at cooler room temperatures. Starter can also be kept in cold or refrigerated conditions for days/weeks/months without feeding. It is re-activated and strengthened by feeding the dough over a number of cycles (called 'stages').
A mature starter is preferred when making bread dough. A starter is mature after reaching optimum fermentation activity, having usually doubled in size (or is just about to fall from its peak) and has a smell that the baker recognises from experience as being 'mature' for their starter (such as a strong lactic or acetic aroma).