The acronym ND stands for a naturopathic doctor, while NMD stands for a naturopathic medical doctor. Both of these names and abbreviations mean the same thing. An individual who has earned this title will generally choose the one he or she prefers based upon where he or she attained their medical education. ND's may take regular board certification exams to become licensed as general practice (primary care) doctors. Their practices are usually integrative, which means they offer mainstream western medical advice.
ND's are not permitted in every state in the US. Since naturopaths are not medical doctors, their services may not be covered by your health insurance. The difference between the services you can receive from the two comes down to the type of education and licensing each receive. If you want someone with medical training and license, look for a naturopathic medical doctor. If you are looking for treatment advice without formal medical knowledge, then a naturopath should be a reasonable option.