A D.O. is a "Dotor of Osteopathic Medicine" They are trained just like regular doctors, but also know skeletal manipulation techniques(think chiropractor). I had a family doc who was a D.O. He did everything like normal, but when I had back pain, for example, he would always perform a manipulation to ease the pain before prescribing any medication.
mamapotter said it all. I've had both MD and DO, medically speaking, they all were great doctors. But, the DO's would always adjust my back, neck and hips whenever I would have problems. The MD would prescribe pills or suggest I see a chiropractor.
Now, whenever I need to search for a new doctor, I look for a DO.
I've found my OB/GYN D.O far easier to talk to and ask questions than the M,D's and Nurses! My DO has a MUCH nicer personality, a nicer "bedside manner," and was more informative (took the time to answer my "silly" questions and give detailed explanations including alternative treatments).
My DO has a more "conservative" approach to surgery and medication, which I agree with!