Simmering involves taking a liquid to its boiling point and then reducing the heat to just below boiling point so that steam bubbles are not formed and the water is not a rolling or raging boil. Boiling point of water (the temperature at which it boils) is usually 100 degrees Celsius at sea level but varies depending upon the surrounding atmospheric pressure. The temperature at which a liquid simmers is widely contested by chefs, ranging from 94 degrees centigrade to 82 degrees centigrade. Cooking at a higher altitude takes longer. Also, boiling takes longer if the surface of the container is wet, or when adding a water soluble substance such as salt or sugar.
edit Effects in Cooking
Boiling is a harsh method of cooking and is not suitable for delicate foods such as fish which will disintegrate. It is suitable for older, tougher, cheaper cuts of meat and for large scale cooking. Vegetables when boiled for too long tend to become soft and mushy. Boiling is a safe, slow and simple method of cooking.
Simmering is a more gentle form of cooking that prevents food from becoming too touch or disintegrating. It is suitable for making stocks or soups, starchy items like potatoes or pastas and many other culinary preparations. It is a rapid and efficient method of cooking.
edit Effects on Health
Boiling water and allowing it to cool before drinking it kills most bacteria, pathogens, and salmonella, and prevents diseases like cholera and dysentery. It also has the added benefit of removing some of the "hardness" in certain kinds of hard water. However, boiling food results in the loss of soluble vitamins to the water (if it is not used and discarded).
Simmering water, which has already been brought to a boil once, is therefore also free of bacteria by virtue of being boiled first.