This is a common question that gets asked often.
I'd like to answer it differently than most answers I hear.
Chiropractors and Osteopathic Practitioners are similar in that we want to help. Same as Physiotherapists, RMT's, Athletic Therapists, Acupuncturists, Naturopathic Doctors etc. We all share a goal and that's to provide services to help improve overall health for people.
Chiropractors use a variety of tools, techniques and are highly trained in their field as are Osteopathic Practitioners. The main difference is they use HVLA (high velocity, low amplitude) thrusts, Osteopathic Practitioners do not. While the training for both Chiropractors and Osteopathic Manual Practitioners are different, I can only substantiate my education and that it meets and exceeds the WHO (World Health Organization) standards with over 4200 hours of educational training during a 4 year program along with an additional 1000 hours of practical clinical training. This in depth program consisted of many courses such as anatomy, physiology (academically and clinically), osteopathic manual manipulation of practical hands-on training that has provided me with palpatory skills to treat gently and effectively.
It's not so much the difference in modality as it is the difference in the practitioner. Finding someone to help you can be challenging and you'll want to be treated by someone you feel comfortable and confident with. Every practitioner, even within the same modality, will have a different approach and style. This is because of their own personality, their background, their interests and clinical experiences helps develop them as a practitioner.
Sometimes words only go so far. The best way is to try it out for yourself!