Sourdough Starter

Starter is simply a growth of the yeast culture which develops first with just plain old bread yeast but it develops it's own unique taste because of the bacteria in the air and water in your own locale.
That is why the sourdough in San Francisco has a unique taste as opposed to a sourdough from Poughkeepsie.

The starter will give you the flavor not necessarily the rise, so for example on the bread recipe before where you let it sit for 18 hours, you would add lets say one cup of the starter to the beginning mix anyway, for the flavor not for really anything else. Ok, a little rise but not much.

You can take out one cup of the dough that has bubbled for 18 hours, that essentially is a starter. It went through all the work to grow and mature, or you can make some starter by scratch to use as starter and not part of the bread reciepe.
Here is a way to make starter, which you could use into the bread reciepe as part of the mix to bring to the table the flavor which keeps on devopoing.

Keep in mind that when you replenish the starter with the milk and flour, use the same kind as before to keep it consistant and not inroduce something you will not be able to recreate.

1 package of active yeast (1/4oz or 7 grams)
1 cup warm (105 -115) water
1 cup warm (105 - 115) non fat milk
2 cups flour

Sprinkle the yeast over water in a bowl, stir until mixed, then mix in the warm milk.
Add the flour, mix to blend, the beat with spoon until smooth.
Transfer the mixture to a 2 quart glass, enamel or stainless steel container. Cover tightly and let stand at room temperatrue (75 to 85) until mixture smells sour. (Usually 24 to 48 hours). Stir it down frquently, it is now active and will be bubbly.
This is starter.

To use it, you stir it down again and remove one cup, add the cup to your bread reciepe. You now need to replenish what you took out. For example if you used 1 cup of hte starer in a reciepe, blend in 1 cup of flour and warm milk, Cover the starter and let it stand in a warm place for several hours or overnight until it is active and bubbly again.
Between uses, refirgerate the starter in a tightly covered container. Always bring the starter to room temperature beore usin git again. Leaving it out overnight before you will need it for baking is usually the easiest.
If you use your starter regualrly, at least ever 10 to 15 days, it should last indefinitely. Some sourdough starters are passed on from one generation of a family to another. I worked in a restaurnat that had a bubbling 5 gallon eartheware crock that was the starter for all the breads they made. It was the job of the new guy to feed it all the time, interesting task to say the least.

I hope that sort of answers the starter question, you will get a hang of it, it's not hard once you figure it out and start using it.
Have fun!