From the American College of Obstetricians and Gynecologists website:
Trust begins with knowing that a doctor is qualified. All ob-gyns have earned their undergraduate college degree and have completed an additional four years of medical school. After receiving their medical degree, ob-gyns complete four years of residency training in obstetrics and gynecology; after this they may elect to become board-certified. Certification recognizes that a doctor has graduated from a recognized medical school, completed an accredited residency program, has an unrestricted license to practice medicine, and has passed extensive exams given by the American Board of Obstetrics and Gynecology.
The site goes on to say:
The initials FACOG after a physician’s name indicate that he or she has met additional criteria to qualify as a Fellow of The American Congress of Obstetricians and Gynecologists. By choosing an ACOG Fellow as your physician, you can feel confident that you will receive the highest quality health care. Choosing an ACOG Fellow as your physician ensures that your ob-gyn is being kept abreast of the latest medical guidelines, treatments, and techniques in women’s health care today.