Graduate medical education
In most industrialized countries, graduating from medical school, with a M.D. or M.B. degree, does not qualify one for independent practice without postgraduate medical education. The exact terminology for this training, which consisted of clinical experience under supervision, with steadily increasing responsibility, varied with the country, and, even within a country, with the institution.
In the U.S., the first year of supervised practice was called an internship, the next several years of specialty training a residency, and additional subspecialty training was called a fellowship. The term externship is sometimes used to describe a period of clinical experience before one graduates from medical school.
With greater trends toward specialization, often beginning immediately after graduation, a trend is to refer to postgraduate year (PGY). The first year after graduation is PGY-1, the next year PGY-2, and so forth.
Clasical internships were "rotating": time was spent in internal medicine and surgery, and, depending on the program, other specialties such as pediatrics or pathology. More recently, if the graduate is clear about future plans, it has been possible to take "straight" internships, such as a full year in internal medicine or surgery.
Again depending on the jurisdiction, it might be possible to get an independent medical license after PGY-1. Realistically, one who did so would have little flexibility, and might not be able to get hospital admitting privileges. Modern postgraduate medical education should be regarded as a minimum of four years.
The length of residencies for primary specialties vary with the organization — usually called a board — supervising the specialty. The minimum is three years. Even within residency, internal medicine, surgery, pediatrics, and emergency medicine are often considered "general clinical"; a specialty, such as radiology, may insist on a year or two of general clinical experience before entering the specific residency. In the other direction, some boards let a graduate start with what might be considered a subspecialty, such as orthopedics starting in PGY-1.
Primary specialty residencies range from three to five years. Again, it will depend on the specialty board as to whether someone could spend PGY-1 on a service, then PGY-2 and PGY-3 in that same service, and then apply for the basic certification in the specialty.
Board eligibility versus board certification
After completion of a residency, there may be examinations to pass, but it is almost always adequate to go into independent practice in a primary specialty. Fellowship training, however, can follow, and to multiple levels of sub-specialization.
For example, cardiology and infectious disease, among others, are subspecialties of internal medicine. Cardiology is a three-year program, which makes one board-eligible in that subspecialty. A graduate, however, may choose to go further; the terms tertiary and quaternary are becoming more common than sub-sub-specialty and the like. A cardiology fellow might choose not to go into general cardiology after PGY-6 or PGY-7, the number of years depending on whether PGY-1 counted against the primary specialty.
Let us assume the graduate took a straight PGY-1 medicine year, so PGY-6 is complete. PGY-7 could be a fellowship in invasive cardiology, which qualifies one to perform intravascular diagnostic procedures such as cardiac catheterization, angiography, and elecrophysiological mapping. Yet another year would be required to qualify in interventional cardiology involving the performing of disease-modifying interventions such as angioplasty, atherectomy, stenting, or electrophysiology ablation or pacemaker installation.
Some graduates decide to combine, perhaps, two primary specialties and a related subspecialty. One might start in pediatrics, complete a residency, and then take two years of general radiology, followed by a year of pediatric radiology. A fair number of secondary, tertiary and quaternary programs allow entry from a variety of specialties. Pain management most often is considered a specialty of anesthesiology, but graduates may well enter a pain management fellowship from pediatrics, psychiatry, or neurology.