Every medical student in the country will one day be faced with dreaded shelf exams after each core rotation. The core rotations include Internal Medicine, Surgery, Pediatrics, OBGYN, and Psychiatry. Once each of these rotations is completed you will sit for an exam based on that topic. They provide motivation to study during your clinical clerkships, and not go straight to sleep when you get home.
Many students use a good question bank and supplement that with a popular study guide. It is also a great idea to check out your local hospital library. Many times they provide these resources to you at no cost, so you do not need to purchase them on your own. These exams are tough, so make sure you are studying throughout the rotation and not waiting until the last week to cram.
Good luck to all those third-years sitting for their shelves, and remember, this is great preparation for Step 2.