A dietitian is an expert in food and nutrition. They give dietary advice, help promote healthy eating habits, develop specific diets for people, and participate in nutrition research. In the U.S., nutrition professionals include the registered dietitian (RD), and the dietetic technician, registered (DTR). They are regulated by the American Dietetic Association (ADA). They must meet strict educational and professional prerequisites and pass a national exam before they are registered dietitians.
Nutritionists are health specialists who are interested in food and nutritional science, preventative nutrition, diseases related to nutrient deficiencies, and the use of food and nutrients to cure disease and ailments.
They also are not regulated. They have varying degrees of education in nutrition – some have very little and some have a masters or phD. Since the term "nutritionist" isn't regulated, anyone can call themselves a nutritionist and dispense nutritional information without having any education whatsoever — they even write books.
Fit's Tips: If you want advice on your diet, food, or nutrition, I'd see a dietitian unless a nutritionist could prove to you how much education they've had.