The purpose of your digestive system is to take the solid/semi-solid food you eat and remove the glucose from it. Glucose is the main carbohydrate all the cells in the human body use to create energy. It would be fair to say glucose is the main fuel your body runs on. The digestive system begins by tearing your food into smaller pieces. Saliva contains an enzyme called amylase, which starts the digestion process of carbohydrates as soon as you put it in your mouth. The food pulp, now called a bolus, is now swallowed down a connecting food pipe called the esophagus, which ends into the stomach.
In the stomach, the somewhat liquefied bolus is mixed with a strong stomach acid, called hydrochloric acid. More digestive enzymes, produced by the liver, pancreas, spleen, and gall bladder, are mixed in and serve as "chemical scissors" to help cut the long chains of amino acids into smaller snippets, from which the glucose may be drawn out. The stomach continues to physically mash the bolus into a more liquefied state by contracting. The food is now in it's most liquefied form, a substance called chyme.
The chyme is now introduced, in small amounts, into the small intestine. The small intestine is a slender pipe that contains thousands of finger-like projections called "villi". The villi have special receptors on them that are like "nutrient vacuum cleaners". Their purpose is to remove all the glucose from the chyme that passes through the small intestine. Everything that is not absorbable is considered waste and is passed along to the large intestine, which is in charge of reclaiming and recycling the water from the waste product. The now more solid waste is stored in the anus, the end of the large intestine, until it is time to be expelled from the system.