I sorta' looked around on the links given above, and most seem to be at least slightly mis-informed about chiropractors, and may make sweeping generalizations about chiropractors based solely on their experiences with OMM.
When I graduated high school, I went to undergrad with the intention of becoming a chiropractor. I had shadowed a chiropractor in my senior year of high school through HOSA, so I think I'm at least somewhat qualified to give my opinion here. While it is true that some chiropractors are pretty far out there with their treatments, i.e. lighting incense and mood music, etc., most chiropractors take their own unique approach. The guy I shadowed used "applied kinesiology", where he used muscle testing (different pressure points on the body that tells him how what exactly needs to be done) in order to diagnose his patients. I've had first-hand experience with this, as my own treatment was what got me hooked on it. Another guy in my area took a more systematic approach, with an X-ray machine in his office that he used as his primary tool of diagnosis.
From all the treatments I've seen them do, coupled with the treatments I've seen with shadowing a DO, I can say that they're pretty similar, particularly in the execution of the actual treatment. However, OMM and chiropractic differ in that chiropractors seem to have more freedom in how they go about diagnosis, while DOs are usually more systematic. Treatment is pretty much the same between the two, but how they reach the same conclusion is where it can, and usually does, differ.
Since the first few years in undergrad though, I've decided to become a DO (and recently getting accepted to TCOM only makes it that much more real!), for several different reasons. But I still have the desire for manipulation, and I really can't wait until I start learning OMM.