Most kinds of cancer are named after the part of the body where the cancer first starts. Colon and rectal cancers begin in the digestive system, also called the GI (gastrointestinal) system. This is where food is changed to create energy and rid the body of  waste matter. After food is chewed and swallowed, it travels down to the stomach. There it is partly broken down and sent to the small intestine. The word "small" refers to the diameter of the small intestine.

The small intestine is really the longest segment of the digestive system. It is about 20 feet long.The small intestine continues breaking down the food and absorbs most of the nutrients. The small intestine joins the large intestine (large bowel), a muscular tube about five feet long. The first part of the large bowel, called the colon, absorbs water and nutrients from the food and also serves as a storage place for waste matter. The waste matter moves from the colon into the rectum, the final 6 inches of the large bowel. From there the waste passes out of the body through the opening called the anus during a bowel movement. 

The colon has four section. Cancer can start in any of the four sections or in the rectum.
The wall of each of these sections (and rectum) has several layers of tissues. Cancer starts
in the inner layer and can grow through some or all of the other layers. Knowing a little
about these layers is helpful because the stage (extent of spread) of a cancer depends to a
great degree on which of these layers it affects.

Cancer that starts in the different areas may cause different symptoms. Colon and rectum cancers probably develop slowly over a period of several years. We now know that most  of these cancers begin as a polyp--a growth of tissue into the center of the colon or rectum.  Polyps are also known as adenomas. Removing the polyp early may prevent it from  becoming cancer.

Over 95% of colon and rectal cancers are adenocarcinomas. These are cancers of the cells that line the inside of the colon and rectum. There are some other, more rare, types of tumors of the colon and rectum, but the facts given here refer only to adenocarcinomas.

Colon and rectal cancer have many features in common and are often referred to together as 
colorectal cancer. They are discussed together here except for the section about treatment. At that point they will be discussed separately.