Ketogenic Diet can be any diet where ketones are expelled by the liver, through the metabolization of fat rather than glucose. Through carb restricions in diet, the body adapts to prefer burning fat for energy. Blood sugar is controled in ketosis as insulin is no longer being release to get rid of blood sugars. By eating plenty of protien and high fat foods, this results in less cravings and feeling full longer.
A diet that will keep your body in a ketogenic state generally restricts carb intake to 0-50g per day. Most people begin with 20g net-carbs (total carbs minus fiber) to get a feel for how to eat. This will quikly cut out junk foods and refined carbs. Whole foods such as veggies, nuts and dairy will be the primary source of carbs. Refined carbs, starches and sugars must be avoided, even from most fruit. Meal focus will be high fat, moderate protien and a side of vegetables. You can check to make sure you are remaining in ketosis with ketostix, found in most pharmacies.
While your body moves from relying on carbs to fat, some time is needed to get used to the new energy source. This results in what is referred to as the "Keto Flu." Headaches, nausea, dizzyness and other flu-like symptoms are common during this transition. The body is flushing electrolytes and water weight. Some salted chicken broth and plenty of water help to speed this processes up. While the 'flu' may only be a couple of days, it takes three weeks to adapt fully. Its important to be strict with the diet during this time. Water is also of huge importance. Once adapted, you will likely see increased and stable energy throughout the day.