You can think of it like this: colorectal cancer refers to cancer of the large intestine. The large intestine itself is actually subdivided. The end part of the large intestine, which is just above the anus, that is the rectum.
But most of the whole part of the large intestine, that is the colon. So, the colon refers to the first 80% bit of the intestine and the rectum is about the end bit which is about 20%. Colon cancer and rectal cancer is a subset of colorectal cancer.
