Digestion involves mixing food with digestive juices, moving it through the digestive tract, and breaking down large molecules of food into smaller molecules. Digestion begins in the mouth, when you chew and swallow, and is completed in the small intestine.

1. Digestion begins before you ever start eating. Glands that are connected to your mouth make extra amounts of saliva as soon as you see or smell food that is appealing to you. Your teeth and tongue take the first steps in battering food into bits. As they are shredding and grinding, more saliva is squirted into the food to moisten and soften it. The saliva contains chemicals called enzymes, which break down the starches in food.
2. The first major muscle movement occurs when food or liquid is swallowed. Although you are able to start swallowing by choice, once the swallow begins, it becomes involuntary and proceeds under the control of the nerves.
3. When swallowing, the mouthful of food makes its way down the food canal, or esophagus, to the stomach. Food does not free fall down to the stomach but is squeezed along by the muscles in the esophagus. At that particular station, there is a ring-like valve, called the pyloric valve, closing the passage between the two organs. As food approaches the closed valve, the surrounding muscles relax and allow the food to pass through to the stomach. This squeezing/pushing action by the muscles is called peristalsis .
4. The stomach has three mechanical tasks. First, it stores the swallowed food and liquid. To do this, the muscle of the upper part of the stomach relaxes to accept large volumes of swallowed material. The second job is to mix up the food, liquid, and digestive juice produced by the stomach. The lower part of the stomach mixes these materials by its muscle action. The third task of the stomach is to empty its contents slowly into the small intestine.
5. The food is now a mashed-up milky liquid. Next it is squeezed into the small intestines. It has its own set of digestive juices for the final breakdown of food. Peristalsis puts the squeeze on the food using the muscles in the internal wall. The muscles contract and relax in the same way that you might squeeze a tube of toothpaste. All this movement makes a lot of noise. This is the grumbling and growling that you can sometimes hear from your stomach. The walls of the small intestines are lined with millions of tiny finger-like projections called villi. The villi absorb the usable parts of the broken -down food into the bloodstream. Finally the food, which is now nutrients and energy, is going to the body.
6. The non-useful parts of the food continue to move into the large intestines. The large intestines absorbs some of the water and salt . The remainder of the material is compacted and then sent out the anus as solid waste or feces, which eventually comes out from the anus.

Here's a brief diagram (Click to enlarge):

How are the
Nutrients Transported?

Most digested molecules of food, as well as water and minerals, are absorbed through the small intestine. The mucosa of the small intestine contains many folds that are covered with tiny fingerlike projections called villi. In turn, the villi are covered with microscopic projections called microvillus. These structures create a vast surface area through which nutrients can be absorbed. Specialized cells allow absorbed materials to cross the mucosa into the blood, where they are carried off in the bloodstream to other parts of the body for storage or further chemical change. This part of the process varies with different types of nutrients.