A medical student must take, and pass three exams in order to become a licensed physician. The test is called the United States Medical Licensing Examination (USMLE).
After 2nd year: Step 1 (test of *all* book knowledge gained in medical school--supposedly the hardest of the three, but I haven't taken any of them yet. I'm taking this in 2 months, yikes!)
Early to mid-4th year: Step 2 CK (clinical knowledge--written test on wards medicine) and Step 2 CS (clinical skills--practical test on physical exam techniques and patient interviewing).
After PGY-1 (Intern year): Step 3 Combination of the above; final test needed in order to legally practice and prescribe medication in the US.
Hope that helps!
-B