* Doctor Who: This refers to the titular character of the long-running British science fiction television series. He's an alien Time Lord who travels through time and space in a time machine disguised as a police box. He's not a real doctor in the medical sense.
* Emergency Room Doctor: This is a physician (MD or DO) who specializes in providing immediate medical care to patients in emergency situations. They work in hospital emergency departments (ERs) or accident and emergency (A&E) departments.
* MD: This stands for "Doctor of Medicine." It's a postgraduate professional degree conferred upon physicians after completing medical school and residency training. An MD is a licensed medical doctor who can practice medicine. An emergency room doctor is *one type* of MD. Many other specialties exist (cardiology, oncology, pediatrics, etc.).
In short: One is a fictional character, one is a job title within a medical profession, and one is a medical degree.