What is ketosis and why do you want to obtain it?
Ketosis: a condition in which the body will turn to fat for fuel and is producing ketones as a byproduct. This change in metabolism occurs when the body no longer has incoming carbs to produce glycogen and after the liver has been depleted of stored glycogen. When either stored body fat or incoming dietary fat is used as fuel it breaks down into ketones.

Ketones can be used as fuel for your body or discarded via your breath or urination. It is the rise in ketone production that means you are in ketosis. Take note that although you are in ketosis this does not mean that you are *only* burning bodyfat. The released ketones may be from either bodyfat or dietary fat.

Why: We want to be in a metabolic state where, without incoming carbs, and without consuming too many grams of dietary fat or protein, our bodies will be burning stored bodyfat as fuel. Positive side effects of being in ketosis are that it can be protein sparing (allowing us to retain lean body mass) and that it can lead to a decrease in appetite for some people.

NOTE: Ketosis is NOT ketoacidosis. There are two types of ketoacidosis: diabetic ketoacidosis (which is due to simultaneous rise in blood glucose and blood acidity) and alcoholic ketoacidosis (which is due to heavy alcohol consumption without food intake). If you do not fall into either of these groups your body should have the feedback loops that will keep ketone production in check. Ketones are produced daily by most people to some degree and are a part of the body's system of survival.

See http://members.tripod.com/~Dietman2/ketosis.html for more on this subject.