This question is asked a lot, and can be difficult to explain clearly.
The role of a Nutritionist is a more holistic approach to help people reach their optimal health through dietary and lifestyle choices. They are not educated in Medical Nutrition, so cannot "prescribe" diets to those in with specific disease or conditions as treatment, whereas a Dietitian is qualified to prescribe such treatment for conditions such as Diabetes, cardio vascular disease and other conditions. Nutritionists have studied a Diploma or Bachelor, and completed hundreds of clinic hours, whereas Dietitians study further to include Medical Nutrition allowing them to prescribe medical nutrition therapies to individuals and groups. Both are accredited, however Dietitians are automatically accredited as a Nutritionist, with Nutritionists being able to work in community healthcare settings not clinical settings such as Hospitals and aged care facilities as Dietitians can.
So it is important to choose a practitioner suited to your individual circumstances and needs for the best outcome for you.
http://www.nutritionaustralia.org/national/resource/nutritionist-or-dietitian for further information.