Food is not in a form your body can use. So it must be converted. And that is what your digestive system does.
Digestion is a process in which food is broken down into substances that are small enough to be absorbed. Food conversion starts in your mouth and ends in your small intestine, where most nutrients are absorbed. Generally, the mechanisms used to digest are the action of chewing, saliva, hydrochloric acid and digestive enzymes. After you chew and swallow, the rest of the digestive process is involuntarily controlled by hormones and nerves.
The first step in the digestive process is in your mouth. Chewing and saliva helps transform food into an easy to swallow hunk, called bolus. Your esophagus transports this mass from mouth to stomach.
Once in your stomach, food is mixed with gastric juices through churning. How long this liquid mixture stays largely depends on the nature of food you ate. Carbohydrates move on the quickest, than protein and fat.
Your stomach slowly empties its contents into the small intestine. In the small intestine food continues to be digested to make it ready for absorption. Digestive enzymes provided by your pancreas and intestine itself helps in this process.
Once the digestive process has broken food down to the appropriate size, it moves through your intestinal wall into the bloodstream. However, fats require the help of bile to make that move. And the leftover is pushed on to the large intestine to prepare it for removal.
Food digestion is a complex process, this health blurb is only meant to provide the basics.