Registered Dietitians (RD) must hold at least a bachelor's degree from an accredited program and pass their state dietetics examination. This is a licensed profession in almost every state.
Nutritionists are not licensed, but generally in most states one has to posses a degree in nutrition in order to call themselves a nutritionist. This is not true of all states, however. Some states such as New Jersey have no regulation whatsoever and allow even people with no training to call themselves "nutritionists", so buyer beware.
The down-side to both of these is their training (ironically). Unfortunately, like the pharmaceutical industry and it's influence over most medical school curricula, both of these degree programs are heavily influenced by "big food" (companies such as McDonald's, Coke, and Kellogg's) . After this article went viral last year people are becoming increasingly aware of big food's influence on both professions formal education and continuing education. Much of what is taught in these programs is the same old, ineffective, potentially harmful and just plain old untrue dogma we've been stuck with for years- fat makes you fat, low calorie and low fat is best, cholesterol is the root of all evil (and heart disease), and the pinnacle of a healthy diet is the vegetarian or vegan diet.
That being said, there are more and more forward thinking RDs now,including this woman in Raleigh. She has an article about why one may wish to choose to pursue an RD above another program, available here.
NTPs also differ from their RD and nutritionist counter-parts in the way that they typically develop their management plans. For RDs and Nutritionists, their recommendations are most often based on the USDA food pyramid (which has long been under scrutiny). NTPs are generally more inclined to prescribe other diets such as the Gut and Psychology Syndrome diet (GAPS), Paleo/Primal, Autoimmune Paleo (AIP), gluten-free, low FODMAPS diet, and the Weston A Price diet rather than the diet the USDA food pyramid outlines.
For more on the USDA food pyramid and its many problems, check out the book Death By Food Pyramid (affiliate link) by Denise Minger.