Ketosis is the process where the body produces ketones to fuel the brain and other organs. Ketones are produced from the breakdown of adipose tissue.
The main fuel the body uses for energy is glucose C6H12O6. Glucose is produced when carbohydrates are broken down by enzymes. For example, sucrose, C12H24O12, is broken down by the enzyme sucrase. Sucrose is common table sugar. When you consume this simple Sugar
some is used to fuel the body, excess is converted to glycogen and stored in the liver, if there is still more then sadly, it becomes body fat. Its been a long time since high biology for me, but that is the basic formula.
When you are fasting or on a low carbohydrate diet the body first calls on its stores of glycogen. When the glycogen is exhausted adipose, or fatty,tissue is broken down. However it does not convert to glucose but become a ketone. Ketosis usually occurs about 48 hours for women and up to 72 hours for men when fasting. People in full ketosis often have breath that smells something like nail polish remover. The body can function while in ketosis for a long time. The one exception is the muscles. Muscles need glucose for fuel. If they do not get it from the diet they can begin to consume themselves. This is commonly called muscle wasting. It is vital not to do strenuous exercise when water only fasting, it can do severe harm to your muscles that can take years to correct. Fasting on juice or a cleansing drink with sweetener reduces the chance of muscle wasting and allows for more exercise.
The obvious advantage of ketosis is that it is obvious that fat is being burned. The bad thing is that ketosis results in a very acidic body. It puts a lot of stress on the kidneys and sometimes the heart. I have read that low carb diets where ketones are the primary fuel while large amounts of protein and fat are consumed are actually more dangerous than fasting ketosis. Perhaps that is because excess protein is also acidifying.
Sorry if I sound like a school teacher, but its not often I get to show off my biology major around here :)