They are a little different, but sometimes you could use either of them in the same situation. Both can mean that something "is about" something else. I think "refer to" is more common if you are talking about written material, and "relate to" is more common if you are talking about concepts or ideas. So, you would probably say: "The following questions refer to the reading passage on page 19." "The following comments are all in reference to the question 'What part did you like best?'" "She referred to the employee handbook to find out about the vacation policy." "When he found out that she hated puppies and kittens, he felt that was related to her fear of intimacy." "He gets lost easily, which might be related to having a bad memory." "All her suggestions were related to preparing for the big test."