We are able to extract energy (calories) from three basic kinds of molecules that are collectively called "macronutrients". These molecular types are fat, carbohydrate and protein.
To put it another way, when we get calories from food, those calories are ultimately coming from a macronutrient (or a combination of macronutrients). As you'll see by following links on the right, each of the three macronutrients have different properties that dramatically influence how they affect human body weight, metabolism and hunger.
The table below summarizes some of the properties of each macronutrient.