Osteopathic medical school curricula closely mirrors those of allopathic (MD) medical schools. The first two years focus on the biomedical and clinical sciences, followed by core clinical training in the clinical specialties. Osteopathic medical school accreditation standards require training in internal medicine, obstetrics/gynecology, pediatrics, family practice, surgery, psychiatry, emergency medicine, radiology, preventive medicine and public health. According to Harrison's Principles of Internal Medicine, "the training, practice, credentialing, licensure, and reimbursement of osteopathic physicians is virtually indistinguishable from those of allopathic physicians, with 4 years of osteopathic medical school followed by specialty and subspecialty training and [board] certification." However, osteopathic physicians also receive an additional 300 - 500 hours in the study of hands-on manual medicine and the body's musculoskeletal system integrated into the medical curriculum.
D.O. means Doctor of Osteopathy. This is a medical degree similar to M.D. The National Institutes of Health Web site has more information about it, but basically both DOs and MDs can practice all types of medicine. This is the NIH Web site: http://www.nlm.nih.gov/medlineplus/ency/article/002020.htm
Actually, users becomethechange and cobraplumber made a mistake. The letters D.O. stand for Doctor of Osteopathic Medicine, not Doctor of Osteopathy (see here: http://www.osteopathic.org/inside-aoa/news-and-publications/media-center/Pages/osteopathic-style-guide.aspx). Otherwise, they are correct, Doctors of Osteopathic Medicine are complete physicians (they attend four years of medical school, go into residencies to specialize in a specific medical field, undergo board certifications and attend fellowships, etc.) and on top of the traditional medical curriculum that overlaps with that learned by Doctors of Medicine (M.D.), they learn a form of manual therapy known as osteopathic manipulative treatment.