Interest surveys are very popular with most of the big websites today. These surveys usually appear when you are buying something or applying for membership to a website. Essentially, websites want to know what your interests are so they can customize the messages they send to you. You can either request information through mail or email. But be sure to pay careful attention to the survey when filling it out.
Some of the bigger sites will also ask if they can give your information to advertisers or other third-parties. This could cause a spam overload in your email if you're not careful. Fortunately, most sites include a box you can check if you don't want to receive anything at all.