Colorectal cancer, also known as bowel cancer, colon cancer, or rectal cancer, is any cancer that affects the colon and the rectum. The American Cancer Society estimate that about 1 in 21 men and 1 in 23 women in the United States will develop colorectal cancer during their lifetime.Doctors aren't certain what causes most colon cancers. In general, colon cancer begins when healthy cells in the colon develop changes (mutations) in their DNA. ... As the cells accumulate, they form a tumor. With time, the cancer cells can grow to invade and destroy normal tissue nearby.