People in the US begin as a medical student
who is in medical school and is not yet a doctor. Other
countries have similar, but different terms for medical
professionals.
After medical school they become an intern, then resident.
Interns are people in their first year of residency. All interns
are residents, but not all residents are interns.
Residents have
officially completed their medical degree and are medical
doctors, but do not have a specialty or license. A resident
still needs to work under an attending doctor. Residency is
where they learn to become a specialist, such as a surgeon.
Residents typically work 3-5 years in specialty training. They
do not get their medical license to practice until after
residency.
Everyone needs a
license to practice as a regular doctor aka attending.
Attendings are people who can independently practice in a
particular area without anyone supervising them. They can be
either a D.O. (Osteopath) or a M.D (Allopath) and both pass the
same board exams.
In order to become a specialist they need to be an attending
doctor first. Fellowship is sub-specialty training after
residency. They are specializing even further, into things, such
as cardiac surgeons must complete four years of college, four
years of medical school, a 5-year general surgery residency and
a 2-3 year specialized cardio or cardiothoracic fellowship.