Crohn's disease can affect any part of the digestive tract (which goes from the mouth to the anus). Most common is Crohn's disease that affects the ileum (the part of the small intestine that joins the large intestine). But Crohn's disease can be in multiple places in the digestive tract at the same time.
This picture shows Crohn's disease that is affecting the ileum and parts of the large intestine (colon), including the rectum. This kind of disease pattern is called ileocolitis.