Yep, been asked ad nauseum, but I'll go ahead and give a basic answer. Feel free to search the forums to see some of the previous threads, but keep in mind that some of the posts are quite mindless.
MDs and DOs have the same basic training (2 years classroom of basic and clinical sciences & 2 years of clinical rotations) and practice very much in the same manner (can specialize in the same areas).
DOs are also trained to do Osteopathic Manual Manipulation.
There are some philosophical differences between the two, but in reality, MDs and DOs usually look very much the same in their practices.
DOs are more likely to choose to go into primary care areas (Family practice, internal medicine, pediatrics, OB/Gyn), probably in part due to the fact that the DO schools tend to emphasize those areas and select students who are already interested in those areas.
Hope that helps a bit.