When a person eats food, the process of how the body gets its nutrients is called digestion. This breaks food down into its respective nutrients.
Carbohydrates get broken down into single units of sugars, protein into amino acids and fats into fatty acids. This process of receiving nutrients actually starts when you chew the food. Then the food travels down into the stomach through the esophagus where it is liquefied by stomach acids. When the food reaches your small intestines, this is where the nutrient harvesting begins.
Most of the digestion and absorption of the food occurs here in your small intestine. Digestive enzymes called lipase, amylase and protease act on fats, carbohydrates and proteins to break them down into their nutrients for absorption.
Once the food has been broken down by the acids into their simple units, they are then absorbed into the blood stream for further chemical changes to make other compounds that the body needs.
Water and small fats cross the intestinal wall easily. Some nutrients such as water and fat soluble vitamins need a carrier to take them across the wall. Other nutrients such as proteins and glucose move across the wall and into the blood stream by themselves but use energy to do so. This is a simple explanation of how the body uses foods as nutrients.