The main and most significant difference is the following:
Anyone, in theory, can call themselves a nutritionist. But in order to be a dietitian (also spelled dietician) you are usually qualified.
Definition: A dietitian is a health professional who has university qualifications consisting of a 4-year Bachelor Degree in Nutrition and Dietetics or a 3-year Science Degree followed by a Master Degree in Nutrition and Dietetics, including a certain period of practical training in different hospital and community settings (in the U.S. 1200 hours of supervised practice are required in different areas). [...]
Definition and Regulation: A nutritionist is a non-accredited title that may apply to somebody who has done a short course in nutrition or who has given themselves this title. The term Nutritionist is not protected by law in almost all countries so people with different levels of and knowledge can call themselves a “Nutritionist”.