Sourdough refers to a bread made by using a starter made of sugar and a natural yeast. If the starter is moved to a new location it will change over time to the local variety of natural yeast.
It has a distinctive tangy or sour taste similar to vinegar.
San Francisco is known for its sourdough breads and sends the starter yeast around the United States for home bakers. The starter has a limited life since it will be replaced by the local yeast variety over time.