What are Shelf Exams?

Just wondering if any one could tell me what Shelf exams are. I've heard them mentioned by the third years and on this forum, and I know at my school they are apparently required, but I'm really not very sure what they are-- can anyone help?
(Of course, since I'm a first year, I really should worry more about next week's anatomy and biochem exams then an exam I'll be taking in third year!)

it’s short for “off the shelf,” as in, your school buys them ready-made from the National Board of Medical Examiners. It’s to contrast them with exams that are written by your own school’s faculty.
NBME puts out shelf exams in lots of different subjects. Larry at UMDNJ took shelf exams during his first year of med school. I took my first shelf exams at the conclusion of my second year, when we had shelf exams in Clinical Medicine and Pathology. I just took a shelf exam to conclude my eight-week rotation in Pediatrics… but I understand that my psychiatry rotation concludes with an in-house exam rather than a shelf exam, so even in third year it still varies.
People worry about shelf exams because they are comprehensive and they may well cover stuff that wasn’t emphasized or even covered in your class. I have found them to be MUCH more… um, well, fun isn’t the word… gratifying to take because they are WELL-WRITTEN. They may be hard but at least you can usually figure out the point of the question, which is more than most of us can say for exams written by many basic science faculty!
so don’t worry about shelf exams. Good luck on anatomy and biochem!
Mary in Virginia