The Keto Diet is gaining some popularity and is similar in spirit to Atkins, Bulletproof, and Paleo. The idea is to eat very low carbs (under 50g or even less than 20g depending on the advisor), a moderate amount of protein and a lot of fat. Fat generally makes up 60 - 80% of intake on these diets. Because most Americans eat so many carbs which are loaded with sugar and addictive, insulin levels are kept high.If this stays high, people become insulin resistant and this leads to the common condition now known as Metabolic Syndrome. That is revealed in the fat around the bellies and related symptoms like high blood pressure and diabetes.
By keeping carb intake very low for several days (or maybe a couple of weeks), your body enters a state of ketosis. Your body will stop using glucose for fuel and will switch to burning fat instead. This is where the diet brings big results.
Some of the Keto books will recommend using MCT Oil or coconut oil as these can help induce a state of ketosis (although some say that you should not use oils).
You can purchase a variety of tests to help you monitor if you are in ketosis.