The full form of MD is Medicinae Doctor. In other words, 'Doctor of Medicine.'
The term 'medicinae doctor' is Latin in it's etymology, and translates to the equivalent of 'teacher of medicine.'
In some countries, this is obtained after achieving a bachelors degree, and is a professional doctorate for training physicians and surgeons.
In other countries, however, an M.D. Is more akin to a PHD (doctor of philosophy) and is in essence a research doctorate. This is commonly found in countries of the European Union such as The United Kingdom, Ireland and India.
In the U.S, for example, admission into medical school is considered to be extremely difficult. In recent years, it is said that only approximately 1/4 of applicants will be granted an offer in to medical school. Even with this considered, some choose to drop out over the daunting pressures and stresses of the program, a long with the time and effort required to procure it (including the bachelors).
I hope this helps.