OB/GYN is the abbreviation for obstetrics and gynecology.  This is the specialty of medicine that focuses on everything related to the medical science of women and everything dealing with the women's body.  Now gentlemen, keep in your pants (LOL).

Obstetrics
Pregnancy, child birth, and postpartum care.

Gynecology
Deals with the female reproduction, which includes the entire reproductive system of the female, including breast, sexuality, and family planning.

Obstetrics and gynecology overlap in most cases.  Doctors in those specialties usually practice both.  When it comes to caring for their patients, they usually do so in a long-term doctor-patient relationship.