Osteopathic Medicine is practiced by Doctors of Osteopathic Medicine (DO) with a philosophy of treating people, not just symptoms or diseases. This has sometimes been referred to as Holistic Medicine, meaning treatment of the whole person, with regard to their bodies and symptoms (just like Medical Doctors), and also with regard to their home, jobs, and environment to make a full assessment of their health and wellness.
Doctors of Osteopathic Medicine take additional courses on diagnosing musculoskeletal dysfunctions in their patients on top of the same types of classes that MDs take, and they are trained to use some forms of Osteopathic Manipulative Treatment (OMT) that is not taught to MDs.
It is one of the two types of study and practices of doctoral level medicine licensed in the US. The other type of medicine is practiced by Medical Doctors (MD), which is often defined as the treatment of symptoms and disease using traditional medical therapies. This is sometimes also referred to as "allopathic medicine", however, it is not a term that is accepted by all MDs, since it was originally coined by critics of traditional medicine and was used with a negative connotation.
MDs and DOs share more similarities than differences in the scope and practice of their work. Both MDs and DOs are licensed to provide identical services to patients including surgery and other specialties. Their studies and training are equal at all levels and in all areas (except as noted above).
They must both pass comparable examinations to become licensed to practice medicine in the states in the US. They are also typically equally paid to perform the same functions and give the same levels and quality of care.