Colonoscopy is a way to look inside the colon, and remove, or take biopsies from polyps or other abnormal tissue.
A flexible long tube (colonoscope), which has a light source and a camera at its' end, is inserted into the rectum, and advanced foward into the rest of the patient's colon (large intestine). The video images of the colon are displayed on a television monitor, as the physician advances the scope through the colon. The doctor looks for abnormal tissue, or anything else that may be unusual. If a biopsy of tissue is required, the doctor has the ability to thread biopsy forceps through the colonoscope, and take a sample of tissue.
In more simple terms, it's like someone is putting a long camera in your butt, and watching the inside of your colon on T.V.
Since this procedure can be quite painful, patients are given IV sedation and pain control.