A starter is a "pre-ferment." That means the yeast, or some of it anyway, has been given an opportunity to run through its life cycle of eating, reproducing and dying. The longer and harder the yeast works, the more of a tang it develops.
There are a lot of different kinds of yeasts used in baking. Some of them are particularly suitable for sourdoughs, because they're so hardy. Many of those are "wild" and given the opportunity will colonize an appropriate dish of food (food for yeast, anyway). Others have been saved and are given away and sold. Those are called "mothers."
Starter is made by mixing a mother with flour and water, and allowing the yeast to run through its life cycle -- feeding as necessary to keep a healthy colony alive. The longer the colony goes, the more sour it becomes. Some of the sourness comes from alcohol the yeast produces as it nears the end of its life cycle.
Usually when we think of starters we think of the kind of sourdough culture one keeps going for years -- continually feeding and using it. But there are other kinds which are more commonly used -- like "poolish" and "biga." Breads based on these pre-ferments aren't as tangy as a real sourdough, but they sure have a lot more going on than a typical American loaf.
These can be given a little extra ripening to get nice and sour. If you want to try a sour bread in the European style, which doesn't involve quite as much effort as a true American sourdough, try this: http://www.cheftalk.com/forums/pastr...-sour-rye.html
It's a good education.
Almost every baker wants to try her or his hand at real sourdough at some point. If you do, I suggest buying a mother rather than trying to start your own from wild yeast. It can be a little hit or miss, and it's no fun wasting two weeks. In the meantime, great mothers can be had for the price of a self-addressed, stamped envelope. Carl Griffith Sourdough Page