So what is a starter?
A culture of naturally occurring yeast and lactic acid bacteria, obtained by the fermentation of flour and water. By refreshing an existing starter with further flour and water we culture this community of wild yeast and lactic acid bacteria, and in managing optimal levels of microbes we can use it to inoculate flour and water to make bread.
At the Sourdough School we use the starter to make a leaven with. The leaven is used instead of commercial yeast. Making a leaven helps to build higher levels of microbes for a more open crumb sourdough.