The philosophy behind the Ketogenic Diet is, when you curt carbs out of your diet, your body has to find an alternative source of energy. If you are taking in enough protein, your body will usually use stored body fat first, if not it may turn to using muscle tissue. The Ketogenic diet is a way to trick your body into thinking it is in starvation mode while your still consuming enough calories from protein and fat to provide satiety and protect your muscle mass from being used as fuel. One way of determining if you are in ketosis is with ketone strips (yes, you pee on them).
What you should eat: Meat, cheese, oils, eggs, very low carb protein products
What you should not eat: Processed foods, sugars, grains, and even fruit and vegetables.
Pros: You can consume your regular amount of calories, or slightly less, so you are not likely to suffer from hunger. Once you reach Ketosis, you can add some fibrous greens back into your diet.
Cons: Very restrictive diet without not being able to eat carbs at all.
So why do it? It is one of the fastest ways to loose fat. Because you can eat as much as you want through the day as long as you stick to the program…no carbs. This style of diet was very popular back 10-20 years ago. Some athletes still use this program to kick start a diet or leaning up phase, then switch to carb cycling once in ketosis.