By default, your body will use glucose from carbohydrates as its main energy source to fuel the body.  On a ketogenic diet, you greatly reduce your consumption of carbohydrates which in turn greatly reduces the amount of glucose you have available to fuel the body.  Your body compensates for this by prompting the liver to produce ketones out of fat (the fats you eat and your body fat stores).  Your body can then use these ketones as an alternative fuel source. 

There are many potential benefits to following a ketogenic diet but the most popular benefit that you see touted in the media, is the potential for body fat reduction and weight loss.  Generally, those wanting to lose weight on a ketogenic diet, will monitor their macro-nutrient and caloric intake carefully.  Keep in mind though, that every person is different, and some may achieve weight loss goals by following a less strict ketogenic diet that is often dubbed as “lazy keto”.  Lazy keto tends to be a less strict form of the ketogenic that follows the general principles of reducing carbohydrates, increasing healthy fats and eating moderate amounts of good quality protein, but tends to be less strict as it usually doesn’t involve tracking these macronutruient amounts but instead relies on guestimating (estimating the amounts you are eating).