A bread starter is a live yeast culture living in a paste of flour and water, or flour and milk. It is used for leavening bread.
There are two ways to get a starter. The first is to make one from scratch. You let a flour batter stand uncovered in your kitchen for several days, and wait for wild yeasts in the air to colonize the batter.
The other way is to get a some starter from someone else, and use it to start your own batch.
There are many recipes for making your own sourdough starter, though purists insist that true starter can only come from other starter, and cannot involve any commercial yeast. You can purchase sourdough starters through mail order.
You have to constantly care for a starter, using it and feeding it to keep it alive.
Never use a metal container or metal implements to store a starter in. Metal seems to inhibit its growth.
1 tablespoon active dry yeast = 2 1/2 teaspoons instant yeast = 3/4 ounce fresh yeast
Freezing will kill starter, but you can dehydrate it. Use starter that has been fed the day before. Spread a very thin layer out on plastic wrap; let sit to dry for two days. Break into flakes, store indefinitely in a dry place. To use, make a paste from a tablespoon of flakes with a tablespoon of tepid water, then add in another cup of water, then a cup of flour. Put in a warm space and let ferment overnight. By the morning, if bubbles have formed, your starter has been successfully brought back to life.