What is sourdough bread?
Sourdough bread does not use yeast to rise. Instead it uses a starter, which is fermented dough, as its rising agent. Hence it is classified as a naturally leavened bread. It normally takes longer for a sourdough bread to rise. Sourdough bread is more flavourful than normal bread. 

The starter is created using flour, salt and water. It has to be "fed" and maintained with fresh flour for the starter to remain alive. Staters can be stored in a cooler environment in order to slow down the fermentation prices.