This question came up a couple of weeks ago. Take a look at this: http://www.italki.com/question/262760

FYI means "for your information." It is usually used when you're giving someone helpful information, rather than when you are asking for information. For example, you could tell your roommate, "FYI, the landlord is coming to inspect the apartment today." I use it interchangeably with "just so you know..."