Residency

What is a residency?

Upon graduation from medical school, you become a "doctor" having
earned the M.D. or D.O. degree. However, this isn't the end of
formal medical training in this country. Many moons ago, back when
almost all physicians were general practitioners, very few
physicians completed more than a year of post-graduate training.
That first year of training after medical school was called the
"internship" and for most physicians it constituted the whole of
their formal training after medical school; the rest was learned on
the job. As medical science advanced and the complexity of and
demand for medical specialists increased, the time it took to gain
even a working knowledge of any of the specialties grew to the point
where it became necessary to continue formal medical training for at
least several years after medical school. This training period is
called a "residency," earning its moniker from the old days when the
young physicians actually lived in the hospital or on the hospital
grounds, thus "residing" in the hospital for the period of their
training.

During residency, you and your classmates practice under the
supervision of faculty physicians, generally in large medical
centers. Many primary care specialties, however, are based in
smaller medical centers. As you grow more experienced, you assume
more responsibilities and independence until you graduate from the
residency, and you are released to practice on your own upon an
unsuspecting populace.

The length of residency programs varies considerably between
specialties and even a little within individual specialties. In
general, the surgical specialties require longer residencies, and
the primary care residencies the least time.

Major medical specialties
Allergy & Immunology
Anesthesiology
Colon & Rectal Surgery
Dermatolology
Emergency Medicine
Family Practice
Internal Medicine
Medical Genetics
Neurological Surgery
Neurology
Nuclear Medicine
Obstetrics & Gynecology
Ophthalmology
Orthopaedic Surgery
Otolaryngology
Pathology
Pediatrics
Physical Medicine & Rehabilitation
Plastic Surgery
Preventive Medicine (including Occupational Medicine)
Psychiatry
Radiation Oncology
Radiology
Surgery
Thoracic Surgery (including Cardiothoracic Surgery)
Urology

Lengths of Some Residencies
---------------------------
All surgical specialties 5+ years
Obstetrics and Gynecology 4 years
Family medicine 3 years
Pediatrics 3 years
Emergency Medicine 3-4 years
Psychiatry 3 years

http://www.faqs.org/faqs/medicine/education-faq/part2/section-4.html