Sarah, The diet is high in fats and low in sugars and carbohydrates. Medium Chain Triglycerides (fats) are common to help with this. MCT oils can be used. When the liver processes fats in the absence of carbs, they are processed into ketone bodies. Only the brain and nervous tissues can metabolize ketone bodies.

The wiki link I posted does a good job of explaining it in detail. http://en.wikipedia.org/wiki/Ketogenic_diet