An osteopath is a physician who is trained in the philosophy of osteopathic medicine. These doctors have a D.O. degree which stands for Doctor of Osteopathic Medicine, as opposed to M.D. for Medical Doctor.
Osteopathic medicine differs from conventional medicine, which is sometimes referred to as allopathic medicine, in that they incorporate a type of manual therapy called Osteopathic Manipulative Medicine (OMM) into their treatment in which they manipulate the spine in order to relieve pain when other methods have not been successful. This is reflective of the philosophical difference between the two types of physicians; osteopaths are trained to treat the body as a whole in the belief that the body is structured with the ability to heal itself while medical doctors treat specific illnesses or symptoms.
Overall, osteopaths and medical doctors in the US are both equally qualified to treat patients and they work in the same settings and provide the same treatments.