There's not too much difference between a dietitian and a nutritionist however the main difference between a nutritionist and dietitian is that only nutritionists may become registered with the Commission on Dietetic Registration (CDR) and may legally declare themselves as a registered dietitian.
Also unlike dietitians the profession of a nutritionist is much less protected under the law.
Both Dietitians and Nutritionists usually help a person to lose weight as they're pretty well knowledgeable in weight loss and nutrition to stay healthy and fit.