What fun you will have!
   Technically speaking , it's an endoscopic exam of the large colon as well as the distal part of the small bowel using a fiber optic camera or a CCD camera. The doctor can see if you have ulcerations or polyps etc. While he is in there he can take a biopsy sample for later diagnosis, and remove questionable lesions.

My experience was: 
  For the day before you have to drink a lot of a special liquid. You get a
prescription for about a gallon of stuff  that cleans your colon out.
Needless to say you can't eat anything and you will be in the bathroom
often.

  Do not cheat this. When I went there a woman was rejected after the IV started and the doctor took a look up her butt and there was some residual "stuff". She had to hang around until the medication wore off and they sent her home. She had to do it all again. Clean that colon!

  The next day with a pure and spotless colon you will go to the doctors medical facility.
The check in and paper work takes a little while. Then they call you in to a room to change into a hospital gown, take your blood pressure and you tell them whatever medications your are taking,  then they get you up onto a stretcher.

  They will start an IV (Intravenous) and sedate you just before they get you into the room and position you on the procedure table for the exam. You are asked to roll onto your side so that your bum is facing the doctor as he sits on a stool beside the table. The doctor usually says hello and how are you doing etc... In front of you you can see the monitor showing all that the rectal probe camera sees. The sedative/pain killer medicine is kicking in just before they insert it. You feel a slight discomfort going in and another discomfort as they turn the corner from descending colon to transverse, etc.

   After that I can't remember some of it as they went in because I was so oblivious from the medication. I do remember when they were slowly pulling it out and I could watch on a monitor. I could see the inside of my colon. Nice and clean. It was interesting but the medication had whacked me out and I was singing "Oh You Beautiful Doll" to the nurses.
Afterwards you have to go to a recovery room for a few hours to come down from the medication. Although, at that time you aren't totally sobered up they can release you to a friend or relative.

   You can not take a taxi or drive home alone. They have that rule for your safety. You have to be accompanied by a responsible adult.

Well, they say the test is good for several years so the fun will be over for
a while. It does not hurt at all.
That was my experience.
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My pal "LD" (who shall remain nameless for confidentiality sake) wrote me an e-mail. He has had several endoscopic experiences.
Here it is:

"ALL LIES....LOL! EVERY DOCTOR IS DIFFERENT SO THE PREP CAN BE DIFFERENT THAN WHAT YOU DESCRIBED BUT, THE RESULTS THERE LOOKING FOR SHOULD  BE THE SAME. YOU SAY IT DOES NOT HURT AT ALL BUT, YOU SHOULD TELL THEM THAT'S NOT ALWAYS THE CASE. IF THEY DO FEEL ANY PAIN WHAT SO EVER  THEY TELL THE DOCTOR RIGHT AWAY. I USUALLY TELL HIM EVEN BEFORE HE STARTS THAT I DON'T WANT TO FEEL A THING. I ALSO TELL HIM THAT I KNOW WHAT HE LOOKS LIKE AND, WHERE HIS KIDS GO TO SCHOOL, PLUS HIS WIFES DAILY ROUTINE.....LOL!"