Ketosis is the metabolic process of using fat as the primary source of energy instead of carbohydrates. This means your body is directly breaking down its fat stores as energy instead of slowly converting fat and muscle cells into glucose for energy. You enter ketosis when your body doesn’t have enough glucose (carbohydrates) available. The prime function of the ketogenic diet is to put the body in ketosis.

Ketones are byproducts of the body breaking down fat for energy that occurs when carbohydrate intake is low.

Source: http://www.perfectketo.com/guide/ultimate-guide-to-ketosis/