Extra qualification to make some extra money for British GPs
The Diploma of the Royal College of Obstetricians and Gynaecologists is aimed at general practitioners who have an interest in providing gynaecological and obstetric services to their (obviously female) patients. The areas that the diploma covers include antenatal services, assistance during labor, contraception, fertility and menopausal problems. This is obvious quite a list which is normally covered by specialised Gynaecologists, but in rural communities around the world these are pretty hard to come by, and that's where the GP with the DRCOG is probably at his/hers best.
The diploma is not particulary hard to get: you have to be able to prove to the college that you did at least 6 months of Obs & Gynae as a Senior House Officer and pass a written and practical exam, a so called OSCE (objective structured clinical examination) which is not really that hard.
After passing it, you can put the coveted 5 letters behind your name and expect to be swamped with menopausal, depressed and pregnant female patients, as your colleagues will tell these patients to see that nice doctor who has a special interest in that sort of thing.