Whether you take the USMLE or the COMLEX you receive a 2 digit and a 3 digit score.

The 3 digit score is comparable across several different administrations of the exam. For example, a score of a 205 in the Summer of 2008 is comparable with someone who also received a 205 in the Summer of 2007. It is the score used by most residency directors when evaluating applicants.

The 2 digit score is not a percentile or a percentage. The 2 digit score is only relevant in the pool with which you took your exam. The NBME (group that administers the USMLE) reports that they are required by some licensing authorities to have a 2-digit score where the minimum passing score is a 75. Because of this the 2-digit score is not comparable between different test administrations. In other words a score of 81 in the Summer of 2008 is not necessarily the same as a score of 81 in the Summer of 2007. Also, on the USMLE, a 2-digit score of 99 correlated with a 234 or above. Certainly there is a difference between a 234 and a 244, but both received a 99 (further diminishing the usefulness of the 2-digit score).

USMLE (Step 1): more info about scores
The USMLE is the licensure exam administered for all medical students (required for allopathic students). It is administered by the NBME.

Most 3-digit scores fall between a 140 and 260. The minimum passing 3-digit score is a 185 and the average is between a 200 and a 220 (standard deviation of 20).

The 2-digit score is not a percentile. The minimum passing score is a 75.

The USMLE does not report percentiles (again the 2-digit score is not a percentile or a percentage).

You may also want to see the 2007 USMLE Performance Data for more interesting information.

COMLEX (Level 1): more info about scores
The COMLEX is the licensure exam required for all Osteopathic Medical Students and is administered by the NBOME. They also release a 3-digit and a 2-digit score.

The minimum passing 3-digit score is a 400 and the mean is a 500. The standard deviation varies between years but the most recent was a 79.

Similarly to the USMLE, the minimum passing 2-digit score is a 75. The most recent standard deviation was a 3.95. The 2-digit score is not a percentage or a percentile.

The COMLEX does not report a percentile.