Sometimes, they are considered the same.
I know when I seen a dietician, on the outside of the door it says 'Nutritionist'. All registered dieticians have a degree in nutrition. You can work at a health food store and be called a nutritionist and not have a degree.
If you get referred from your doctor, they are more then likely going to be the same people, but with a degree. It just depends on what you want to call them. Dieticians work on dieting, nutrition, healthy living and ways to eat good for you food.
Hope that helps.
(seeing the word nutrition a few times, totally makes me think it's spelled the wrong way lol)
Edited to add: My cousins son is autistic and he sees a dietician regularly to help him and his parents get him the right food and ways to make the right food so he eats it.