I have a hard time understanding starters too, but there can be different kinds. From what I know a preferment can be used as starter and can be classic preferments (poolish, sponges, bigas, etc.. A poolish usually is equal amounts of water and flour with some commercial yeast added. They can be added when they hit the break point or they can be left to bubble and then cold fermented before they are incorporated into the final dough. A natural starter (started with rye flour and then fed with water and flour, until it develops, with only using “wild yeast” from the area you are in. This is where Toby and other members helped me with a natural starter. http://www.pizzamaking.com/forum/index.php/topic,10008.msg87117.html#msg87117
A starter can also be one you buy and activate, like I just did with the help of others at http://www.pizzamaking.com/forum/index.php/topic,11578.msg106188.html#msg106188
This is copied from the glossary. http://www.pizzamaking.com/pizza_glossary.html#S
PREFERMENT: A partial preparation of flour and water and yeast (naturally-occurring or commercial), that may or may not include salt, that is left to ferment and mature before incorporating into the final dough. The ingredients and period of fermentation are controlled to achieve the desired leavening power and maturation (ripening) before incorporation into the final dough. The preferment can be fermented at room temperature or under refrigeration, or a combination of both. Depending on its final intended use, it can take a liquid form, semi-liquid form (like a batter) or it can be stiff and dough-like. The benefits from using a preferment include a strengthened gluten structure, a shortened overall production time, and superior crust flavor.
I don’t know if this helps you are not to understand what starters are.