So then, a wild starter is different from a regular starter in that it contains ... surprise, wild yeast. This is as opposed to commercial baker's yeast (also brewer's yeast) in other types of bread. It is apparently widely misconceived that wild yeast comes right out of the air -- I say apparently because I didn't even know that wild yeast was a thing, let alone a thing to be misconceived, until a few weeks ago. The yeast actually lives right there on the flour (particularly rye flour, particularly organic rye flour) and given some time to grow and ferment with regular feedings (i.e., adding more flour) it will turn itself into a little self-sustaining ball of bread-rising power. The really cool implication is that this bread is literally made from flour, salt, and water only (although the complete truth is that the starter began life in half a cup of pineapple juice to stave of leuconostoc bacteria that can sometimes, and on my first attempt did, delay the starter a few days)
All credit to Bread Baker's Apprentice
by the way for this information. So another interesting thing is that it's actually bacteria and not the wild yeast that makes the bread sour. San Francisco sourdough is so distinctive because it contains a type of bacteria that lives only in that hallowed city (Lactobacillus sanfranciscensis believe it or not). The reason the wild yeast is important is that it can live in the acidic environment that the bacterias thrive in, and live long enough for the bacteria to grow.