Colon cancer, colorectal cancer, and rectal cancer are different names for what essentially is the same disease. Colon cancer is the third most common type of cancer among American men and in women and is the third leading cause of cancer-related deaths (after lung cancer) in the United States. A few facts:

• Most cases of colon cancer occur in men and women over the age of 50.
• If the disease is diagnosed at an early stage, there is a good chance that it can be cured. In general, the more advanced the cancer is (the more it has grown and spread), the less chance there is that treatment will be successful. However, even in these cases, treatment can often slow the progress of the disease.
• People over the age of 50, as well as those who are considered at high risk for colon cancer, should be screened on a routine basis so that the disease can be detected at an early stage. Get more facts about colon cancer. 

Colon Cancer and the Gastrointestinal System
Colorectal is a word that means 'the colon and rectum', which are parts of the gastrointestinal (GI) tract. The GI tract starts at the mouth and ends at the anus. When you eat or drink, the food and liquid travel down the esophagus into the stomach. The stomach churns up the food and then passes it into the small intestine.

The small intestine (sometimes called the small bowel), which can be up to 20 feet long, is where food is digested and absorbed. Undigested food, water, and waste products are then passed into the large intestine (sometimes called the large bowel). The main part of the large intestine is called the colon, which is about five feet long. This is split into four sections, the ascending, transverse, descending, and sigmoid colon. Some water and salts are absorbed into the body from the colon. The colon leads into the rectum, which is the last eight to 10 inches of the large intestine. The rectum stores stools before they are passed out of your body through the anus.

Cancer Defined
Cancer is a disease of the cells in the body.

The body is made up from millions of tiny cells. There are many different types of cells in the body, and there are many different types of cancer which can occur in different types of cells. What all types of cancer have in common is that the cancer cells are abnormal and multiply (grow) at an out-of-control rate.

A malignant tumor is a lump or growth of tissue made up from cancer cells which continue to multiply. Malignant tumors can invade and damage nearby tissues and organs.

Malignant tumors may also spread to other parts of the body. This happens if some cells break off from the first (primary) tumor and are carried in the bloodstream or lymph channels to other parts of the body. These small groups of cells may then multiply to form 'secondary tumors (metastases) in one or more parts of the body. These secondary tumors may then grow, invade and damage nearby tissues and organs, and spread again.
Some cancers are more serious than others, some are more easily treated than others (particularly if diagnosed at an early stage), and some have a better outlook (prognosis) than others.

Cancer is not just one condition. If you are diagnosed with cancer, it's important to know exactly what type of cancer has developed, how large it has become, and whether it has spread. This will enable you to get reliable information on your treatment options and prognosis.

Colorectal Cancer
Colorectal cancer is a cancer of the large intestine. It is sometimes called colon cancer or bowel cancer. It is one of the most common cancers in the United States. Colorectal cancer can affect any part of the large intestine, but it most commonly first develops in the lower part of the descending colon, the sigmoid colon, or rectum.

In most cases, colorectal cancer develops from a polyp which has formed on the lining of the large intestine.  Sometimes colorectal cancer begins from a cell within the lining of the large intestine which becomes cancerous.

As the cancer cells multiply they form a tumor, which can invades deeper into the wall of the intestine. Some cancerous cells may break off into the lymph channels or bloodstream. The cancer may then spread (metastasize) to nearby lymph nodes or to other areas of the body – most commonly the liver and lungs.

Polyps and Colorectal Cancer
A bowel polyp (adenoma) is a small growth which sometimes forms on the inside lining of the intestine. Most bowel polyps develop in older people. Polyps are benign (non-cancerous) and usually cause no problems.

Sometimes, however, a benign polyp can turn cancerous. If this happens, the change usually takes place after a number of years. Most colorectal cancers develop from a polyp which has been present in the large intestine for anywhere from five to 15 years.

Causes of Colorectal Cancer
A cancerous tumor starts from one abnormal cell. In the case of colorectal cancer, it is a cell in a polyp which is the most common site for a cancerous cell to develop. While the exact reason why a cell becomes cancerous is unknown, researchers believe the change occurs when something damages or alters certain genes in the cell. This makes the cell abnormal and multiply in an out-of-control fashion.

Risk Factors
Although colorectal cancer can develop for no apparent reason, there are certain risk factors which can increase the chance that colorectal cancer will develop. These include:

• Aging – colorectal cancer is more common in older people. Most cases are in people over the age of 50. 
• Heredity – if a close relative has had colorectal cancer, you may be at increased risk for the disease.
• Other conditions:
• If you have familial adenomatous polyposis or hereditary non-polyposis colorectal cancer. (These are inherited disorders.)
• If you have ulcerative colitis (a condition of the colon) for more than eight to 10 years.
• Obesity.
• Lifestyle factors: lack of exercise; low intake of fruits and vegetables and high intake of red meat; high alcohol consumption; and smoking.

Preventive Factors
There is a reduced risk of developing colorectal cancer in: 

• Women who take hormone replacement therapy (HRT).
• People who eat a lot of fruit and vegetables.
• People who take anti-inflammatory medicines (such as aspirin) for other conditions.

Symptoms of Colorectal Cancer
When colorectal cancer first develops and is small, it usually causes no symptoms. As it grows, the symptoms which develop can vary, depending on the site of the tumor. The most common symptoms are:

• Bleeding from the tumor. You may see blood mixed up with your feces (stools or diarrhea). Sometimes the blood can make the feces turn a very dark color. The bleeding is not usually severe and in many cases the bleeding is not noticed because only a small amount of blood is mixed with the feces. However, small amounts of bleeding which occur regularly can lead to anemia which can make you tired and pale.
• Passing mucus with the feces.
• Changes from your usual bowel movements. This means you may pass feces more or less often than usual.
• Bouts of diarrhea or constipation.
• A feeling of not fully emptying the rectum after a bowel movement.
• Abdominal pains.

Growth and Spread of Tumors
As a tumor grows in the intestine, symptoms may become worse and can include:

• The same symptoms as above, but more severe.
• You may feel generally unwell, tired, or lose weight.
• If the cancer becomes very large, it can cause a blockage (obstruction) of the intestine. This causes severe abdominal pain and other symptoms such as vomiting.
• Sometimes the cancer makes a hole in the wall of the intestine (perforation). If this occurs, feces can leak into the abdomen.

All the above symptoms can also be caused by other conditions, so tests are needed to determine whether or not you have colorectal cancer. If colorectal cancer spreads to other parts of the body, various other symptoms can develop.