aww meg, no problem my friend. 🙂 i'm sorry, i had no idea what they were either before med school, i hadnt even heard about them. sorry! 😳
ok, so the national board of medical examiners (NBME) are the ones that make steps 1, 2, and 3. the NBME is basically to med students what the AAMC is to premeds. the NBME offers subject exams to medical schools that are called "shelf exams" by med students. those subject exams are suppose to model the format and content of how that subject will be tested on the steps. at my school we only use shelf exams during year 3 for pediatrics, OB/gyn, family medicine, psychiatry, internal medicine and surgery. Those are the only clinical subject exams offered by the NBME and we take them all. Our school uses our grade from those exams and factors that into our rotation grades. So my peds shelf grade will be a big chunk of my peds rotation grade. In a way I love these things because they force you to read the review books and answer USMLE questions from the beginning of 3rd year. All the clinical shelfs I'll take this year are basically identical to the content and style of the USMLE step 2 that I will take next summer.
Some schools like EvoDevo's at San Antonio use shelf exams for the basic sciences as well, so they take the shelf for Micro, Pharm, Physiology, Path, etc.... Those are similar to the content and style of step 1. My school didnt use them because we are on a systems based curriculum for the first 2 years and those shelfs are broken up by each individual science.
Only the USMLE Step 1, 2 and 3 are used for licensure. The shelves are just practice subject exams in a way, except that a lot of schools like mine use them as final exams pretty much so they are very important.