USMLE (United States Medical Licensing Examination) is a three part exam required from any doctor wanting to practice medicine in the USA.
The USMLE is supported by the FSMB (Federation of State Medical Board) and NBME (National Board of Medical Examiners).
Medical Physicians can only be licensed doctors in the US post completion of the USMLE.
All three stages of the exam correlate to each other. Doctors are licensed once they complete all three stages.
Step 1 and 2 are can be taken prior to graduation and the Step 3 test upon receiving your medical degree.