Nutrition is the science of food and it´s effects on health. Nutrients supply the body with all the important substances it needs for growth, temperature regulation and the maintanance of it´s physical and mental functions.
There are six essential nutrients, food is composed of:

The body doesn´t really care, what kind of food provides the energy. It can be taken from all macronutrients. That´s why food is quite exchangeable. Looking across different parts of the world, you´ll find countless food compositions, all providing us with the same nutrients.
It´s important to know and understand what you kind of food you consume and how it effects your body and mind. You are what you eat.