Generally, you must include in gross income everything you receive in payment for personal services. In addition to wages, salaries, commissions, fees, and tips, this includes other forms of compensation such as fringe benefits and stock options.
Internship comprises six months of medicine and six months of surgery. A minimum of two months and a maximum of three months may be spent in a sub-speciality, including emergency medicine, general practice, obstetrics & gynaecology, paediatrics, psychiatry, anaesthesia and radiology.
So again, internship is simply the first year of residency. Residency can be anywhere from 2 more to 6 more years, depending on the specialty you choose. Many super-specialties have further training after residency, called fellowship - these can be 1 to 3 years in length.