The most important thing to understand is the three macronutrients that make up our food. Protein, carbohydrates and fats are the fuels that our bodies can convert to calories and burn for energy. They behave very differently and have different chemical properties. One way to illustrate the difference is to look at the calories per gram of each macronutrient. Protein carries 4 calories per gram.
Protein is found in meat, dairy products and beans.
It is also present in starches and veggies in smaller amounts but these are incomplete proteins. Carbohydrates carry 4 calories per gram. Carbohydrates are your primary fuel. Found mostly in grains and vegetables. Fats are found mostly in meat, dairy products and baked and process foods.