There are four nutritional macronutrients or "macros": Fat, Protein, Carbohydrates, and Alcohol. On a Low Karb diet, you try to divide what you eat over the course of the day into 70-80% healthy fats, 15-25% proteins, and 5% carbohydrates
On keto, you measure your carbs in terms of "net carbs" which is your total carbs when you subtract the fiber and sugar alcohol of every product.
Ketosis happens when your liver creates fat-eating "ketones" to creating energy when there are no carbs present. By starving your body of carbs, your body produces more ketones and you'll burn more fat.
On the keto diet you often feel a consistent energy throughout the day, without the highs or crashes. This is often accompanied by a sense of mental clarity and positive moods. It sounds weird, but get into ketosis and you'll know exactly what we're talking about.