Sourdough breads use a fermented batter-like dough starter to make them rise and enhance their flavour. A portion of the sourdough starter is mixed with the bread's ingredients, while the remainder is kept and 'fed' with more flour and water to use in future batches.
A sourdough starter can survive for years if looked after. Don't forget to name your sourdough. Some say it'll help to preserve it even longer. The oldest sourdough on records can be found in Canada and is over 120 years old. 'His' name is Youkan.