Leavened bread is bread that has risen, puffed with gasses that creates a cellular network of pockets throughout the dough. These gasses are produced by fermentation, a process whereby an organism, in this case one of the Saccharomyces strain of fungi we know as "yeast", releases carbon dioxides as it assimilates and uses the sugars present in the dough. Bacteria can also ferment sugars, and are often present together with yeast when fermentation occurs.
Yeast might be found on the surfaces of grains such as wheat, or on the skin of fruits rich in sugars, or in a spoonful of an already fermenting mixture. When these are mixed with fresh water and flour, the yeasts will slowly multiple and ferment. Some bacteria will give the leaven its distinctive sour aroma and taste - as with the bacterium Lactobacillus Sanfrancisco, which has been isolated by scientists, and to which the remarkable bright flavour of the San Francisco sourdough is attributed.
Adapted from
Dan Lepard:
The Handmade Loaf (Mitchell Beazley Food)
#Day 1:
Mix all the ingredients in a 500 ml jar or similar container. Cover and leave at room temperature (approx. 20C/68F) for 24 hours.
#Day 2:
When you open the jar, you'll see that there is a glossy, watery film on the surface, with silt at the bottom of the jar as some separation has occurred. If you smell the contents you may perceive a hint of fermentation. Add the water to the jar and then stir in the flour. Cover and leave again at room temperature (approx. 20C/68F) for 24 hours.
#Day 3:
When you open the jar, you'll notice tiny bubbles like those that form on the inside of a Champagne glass on the surface. These show that the fermentation, caused by the yeasts and lactic acid bacteria (lactobacilli), is well under way. Add the water to the jar, stir well to combine, then add flour and stir again. Cover and leave again at room temperature (approx. 20C/68F) for 24 hours.
#Day 4:
Replace the rye flour with 125 grams of strong white flour if you need a white leaven. 
By this time the froth of fermentation should be beginning, though the hint of acidity in the aroma is rather vague. Discard three-quarters of the mixture. Add the water and stir well. Remove the raisins by pouring the mixture through a strainer (I kept the raisins), then put the liquid back to the jar. Add the flour and stir again. Cover and leave again at room temperature (approx. 20C/68F) for 24 hours.
#Day 5:
Replace the rye flour with 125 grams of strong white flour if you need a white leaven. 
The fermentation should be clearly evident, and the aroma starting to become acid. Discard three-quarters of the mixture. Mix in the water, and then stir in the flour. You should have a thick paste now. Cover and leave again at room temperature (approx. 20C/68F) for 24 hours.
#Day 6 Onwards:
The mixture is bubbling and fully activated. Each day as you remove some leaven for baking, replacing it with an equivalent amount of flour and water, the aroma of the leaven will become stronger and more sharply acidic.
To keep the starter activate, alive and healthy, you will have to feed them with flour and water after discarding three-quarters of the mixture. (or using them to bake bread). Dan Lepard suggests to keep the refreshment slightly heavier on flour than water, as this slows the fermentation and stops the leaven rising and falling too quickly. It's a good idea to continue feeding and refreshing it for at least one more week before you start making bread with it.
:.