The ketogenic diet is a high fat, moderate protein, and very low carbohydrate diet.
Through the restriction of carbohydrates, it re-programs your metabolism to burn your fat stores for fuel rather than using the glycogen stored in your muscles.
The low-carb intake helps to keep your blood sugar levels from spiking. This allows your fat cells to begin releasing fatty acids, which your liver will then breakdown to produce what are called "ketones."
Ketosis is the physiological state that is achieved when your body breaks down ketones for energy rather than the glucose from carbohydrates.
Benefits (other than weight loss) include increased energy, improved performance, improved cognitive function, better digestion, mental clarity, and slowing down of the aging process.