Doctor (title)

A Doctor, more correctly called a physician, is the health profession of curing or easing the ailments of others.

Doctors in all countries must pass through many years of training to qualify.

United Kingdom & Commonwealth

Note that in the UK and Commonwealth countries following the UK system, not all "doctors" are Doctors of Medicine. The academic doctorate in Medicine is a higher doctorate which is only earnt by those who have made a significant academic contribution to medical science. Note also that surgeons are referred to as "Mr", not "Dr", unless they are also Doctors of Medicine.

In the UK a doctor's training normally follows this path:

Degree level Preclinical - Doctors must study medicine in university or medical school for two to three years 'preclinical' (meaning little patient contact). However following recommendations by the British Medical Association (BMA) many universities are following a 'Problem-based learning' approach, which stresses basing the studies around actual patient cases.
Clinical - This time is spent in a teaching hospital and typically lasts two or three years. After this is completed the student doctor is awarded a Bachelor of Medicine (BM or MB) and Bachelor of Surgery (BChir or BS). He/she is now entitled to use the honorary prefix of "Dr.", although he/she is not recognised as a "doctor" in the academic sense of the word (see Doctorate).
House Officer (HO) - At this stage the student is allowed provisional registration as a junior doctor, but must complete two, six month periods as a house officer in a hospital.
Senior house officer (SHO) - This lasts from anything between two to seven years depending on the specialty chosen. The doctor is now officially registered and must complete the time in a clinical position in a hospital.

At this stage the doctor can choose becoming a General Practitioner (GP), other community doctor, or a hospital doctor. The vast majority in the UK work as the former, who diagnose illnesses and refer patients for further examination by specialists if necessary. 90% of all patients are managed by their GP without the need for further referral. Hospital doctors can be promoted from SHOs to Registrar, and eventually to Consultant.

United States

In the United States and countries following the U.S. method, the path to a medical degree is somewhat different.

Admissions: Persons posessing a Bachelors degree [Bachelor of Arts (B.A) or Bachelors of Sciences (B.S.)]--a four-year post secondary degree usually earned at an accredited college or university--may apply for admission into medical school. Admissions criteria include overall performance in the Undergraduate years, performance in a group of courses specifically required by U.S. medical schools (biology, chemistry, organic chemistry, physics, and sometimes calculus), score on the MCAT (medical college admissions test--a national standardized test), application essay(s), and interview.
Medical School: Once admitted to medical school, it takes four years to earn an M.D. or doctorate of medicine degree. The course of study is divided into two roughly equal parts. Preclinical study generally comprises the first two years and consists of classroom and laboratory instruction in subjects such as anatomy, biochemistry, physiology, pharmacology, microbiology, pathology, and neurosciences to name a few. Once the student successfully completes preclinical training, he or she moves on to the clinical portion. This usually occupies the final two years of medical school and takes place almost exclusively on the wards of a teaching hospital. The students observe and take part in the care actual patients. Rotations on clinical services such as internal medicine, surgery, pediatrics, obstetrics/gynecology, and psychiatry are the basic parts of this curriculum but many specialty electives may be chosen as well. On earning a M.D., one can be called Dr.
Internship: During the last year of medical school, students apply for postgraduate residencies in their chosen field of specialization. These are more or less competitive depending upon the desirability of the specialty, prestige of the program, and the number of applicants relative to the number of available positions. All but a few positions are granted via a national computer match which pairs an applicant's preference of position with the programs' preference for applicants. The first year of any residency is known as "internship". Completion of this year is the minimum training requirement for obtaining a license to practice medicine in the U.S.
Residency: Each of the specialties in medicine has established its own curriculum which defines the length and content of residency training necessary to practice in that specialty. Programs range from three years after medical school for internal medicine to five years for surgery to eight or nine for neurosurgery. Each specialty incorporates an internship year to satisfy the requirements of licensure. All specialties hold a board exam (either written or written and oral) at the completion of training in order to confer "Board Certification" in that specialty.
Fellowship: Certain highly specialized fields require formal training beyond residency. Examples of these are Cardiology, Endocrinology, Oncology after Internal Medicine; Cardiothoracic Surgery, Pediatric Surgery, Surgical Oncology after General Surgery to name just a few. There are many others for each field of study. The training programs for these fields are known as fellowships and their participants are "Fellows" to denote that they already have completed a residency and are "Board Eligible" or "Board Certified" in their basic specialty. Fellowships range in length from one to three years and are granted by application to the individual program or sub-specialty organizing board.
Attendings: The physician or surgeon who has completed her or his residency and possibly fellowship training and is in the practice of their specialty is known as a Attending. These are the physicians who may independently care for patients and are the final arbitors of care. They are responsible for all care decisions and may bill for their services.

However medicine is an extremely varied profession and lots of options are available. Some doctors work in pharmaceutical research, Occupational medicine (within a company), Public Health medicine (working for the general health of a population in an area), or join the armed forces.

The term Doctor is also correctly used to describe someone who holds a Doctorate - a higher academic degree. 'Doctor of Philosophy' (PhD) is the most common of these.

Some physician also hold PhD's, typically those who go on to take research as a career.