The keto diet is popular right now and basically involves eating primarily a lot of fat with a limited amounts of proteins and almost no carbs at all. With the Keto diet you are eating less than 50 grams of carbs a day, so of course in the short-term you're losing weight because you're not getting as many calories. However, I found research studies that shows a lot of the weight people lose initially is water weight. Sugar stored in your body is bound with H2O so when you start cutting the carbs your body grabs that sugar and release the water. So you think you're losing fat but in actuality most of it is water weight.
Plus, this isn't very easy to do as your body needs carbs for energy. So if you are active at all you will find yourself running out of steam. Another bad part about the keto diet is that you're basically depriving your body of the high amount of protein in order to build and maintain muscle mass. And because you're not taking quite enough protein as well as eliminating a lot of carbs you are potentially sacrificing a lot of muscle mass.
A lot of people who have been on Keto initially lost because of the water weight. However, when they go back to eating more carbs, they put the pounds right back. The bottom line is that it is a short-term type weight loss plan that has limitations, but definitely not a healthy lifestyle for burning body fat and maintaining muscle mass.