DO is a doctor of osteopathy. MD is a medical doctor. It's not very straightforward as to the differences though.

Both go through an almost identical training - essentially the same classes the first two years in basic sciences, and then clinical rotations the last 2 years. The only difference is that DO school includes a course on "manipulation" - which, as far as I understand, is similar to chiropractic manipulation, but different as far as the intensity of the manipulations and the range of motion of the manipulations. I'm not aware of any scientific evidence that convincingly shows it is effective.

DO schools are generally easier to get into. Both schools can go into residency programs, but graduates of DO schools generally have a tougher time matching into the more competitive fields.

In the end, both are able to practice medicine in the same way - some osteopaths do include manipulation in their treatment, some don't. Osteopaths are more likely to be in primary care medicine, possibly because they are less likely to match into the more competitive subspecialties.

I am in a DO program. The information Dr. M. has provided is mostly spot-on. In some cases, it is easier to get into DO schools than MD schools, although it certainly isn't easy. The MCAT is the same. The prerequisites are the same. The education is the same, although we also add in the manipulation courses.

The efficacy of osteopathic manipulation has been demonstrated in several areas. One of my colleagues, a DO obstetrics resident in an MD residency, recently published on manipulation treatments helping lower back pain for obstetrics patients, and is currently presenting his research at both MD and DO conferences. I can provide references, if you are interested.

Overall, DO schools definitely place an emphasis on primary care education. However, since DOs are eligible for both MD and DO residencies, all of the specialties are available... from the super-sexy surgical specialties, to anesthesiology, to whatever-the-heck-you-want-to-do. I am of the opinion that early medical training as a generalist will make a specialist all the more effective as a physician down the road.

Good luck with whatever you decide to pursue!