What is the difference between a doctor and a midwife?

Overall training: A doctor has usually a college degree, 4 years of medical school and 4 or more years of residency and perhaps followship years of training in obstetrics. A midwife, depending on certification has some apprenticeship with another deliver (lay midwife) to more formal training. Could be a nursing degree plus midwifery school. A physician has much more formal training inmedicine..

Education & training: An OB MD attends a 4 year medical school and then attends 4 years of post graduate residency training to become eligible to be certified by the American Board of Obstetrics and Gynecology (ABOG). A written exam and, if successfully passed, an 3 hour oral examination is given to a candidate. This is a rigorous process that insures reasonable knowledge acquisition of the discipline to provide care.

Depends on location: I believe in my state a basic lay midwife needs only an informal education as an apprentice with another midwife for a minimal number of hours and a half dozen or so supervised deliveries. There are certified nurse midwives with formal college/hospital education that often practice with OBGYN backup. OBGYN's have a minimum of college/med school and residency before they practice.