Gyn vs gyn oncology. A gynecologist takes care of all ladys and health issues related to say "female issues": annual pelvic exams, pap smears, helping with birth control, treating stds, and helping lady's make it through menopause. Gyn oncologist treats ladys who have developed cancer of the cervix, uterus, vagina, vulva etc.
Additional training. Gynecologic oncologists are ObGyn doctors who spend an additional 3-4 years of training specifically to treat gynecologic cancers like uterine, cervical, ovarian and vulvar. They have a lot of additional surgical training, and are trained in using chemotherapy, "biologicals", hormones against cancer and radiation as well. It is a board certified sub-specialty above and beyond ObGyn certification.