What is a calorie?
A calorie is a unit of energy.
The proteins, carbohydrates, fats, and alcohol in the food and beverages that we consume each have a different amount of energy needed to break them down. Protein and carbohydrates require 4 calories per gram. Fat requires 9 calories per gram, and alcohol requires 9 calories per gram.
Typically, any food that is not burned by the body is counted as "extra" calories and stored as body fat. This is why "counting" calories is popular among those who are on a diet.