Medical school grading is a lot different from what you experienced as an undergraduate student. While in professional school, the goal is to graduate competent physicians who can deliver quality health care. Therefore, students are expected to work together and a variant of the pass/fail scale is used to achieve this outcome.
Most medical schools will rank everyone based on academics from first to last. This makes it easy for you the student, and residency directors to know if you are in the top 20% of the class or bottom 20%, since you will be given an exact number. Essentially, there is a need to know how well you are doing, while also limiting competition with your peers.
The Medical School Performance Evaluation, also known as the Dean's letter is a comprehensive summary of your overall performance. You should be aware, that although you may not be receiving a letter grade you still have a duty to excel in your courses and clerkships.
After receiving your degree you will need to gain admission to a residency program and residency directors will review your performance to determine your fitness for their program and/or specialty.