Obstetrics and Gynecology are two distinct medical specialties that in general deal with the female human body. There have been numerous changes and growth in these areas over the years that practitioners of the two, in modern times, are actually trained together. That is why more often than not they are normally merged as a single specialty. You probably have seen them numerous times abbreviated as OB/GYN, O&G or OBG. Even though they are grouped as one specialty, there are differences.
Obstetrics in a nutshell is the care of pregnant women. It is concerned with the care of women and their unborn children during pregnancy and during childbirth. It is also concerned with the pre-natal health of the fetus and finally the post-natal care of the mother to ensure she is recovering well. Gynecology, on the other hand, is simply an umbrella term encompassing the treatment of any disorder or disease that is prevalent in the female reproductive system. This would include the organs of the uterus, cervix, ovaries, vagina. Since none of these live in a vacuum, gynecology also involves the bowel, bladder, and other portions of the GI and urinary systems.
A doctor who has specialized in gynecology is referred to as a gynecologist while one that has specialized in obstetrics is known as an obstetrician. To be allowed to practice both, one needs to undergo intensive training that generally takes 4 years of residency on top of having a medical degree. The two fields normally intertwine and 90% of gynecologists also deliver babies, hence ob/gyn. Over time, some ob/gyns stop delivering babies, either to spend more time in the office or with their families (no babies means no call!) These gynecologists can diagnose a pregnancy, but then refer to obstetricians for further care.