FYI is a common abbreviation of for your information (or interest, in some interpretations).
"FYI" is commonly used in e-mail, instant messaging or memo messages, typically in the message subject, to flag the message as an informational message, with the intent to communicate to the receiver that he/she may be interested in the topic, but is not required to perform any action. It is also often used in informal and business spoken conversations, sometimes sarcastically.
Among Internet Standards, FYIs are a subset of the Request for Comments (RFC) series.
The following description is taken from FYI 1, the FYI on FYI:
The FYI series of notes is designed to provide Internet users with a central repository of information about any topics which relate to the Internet. FYIs topics may range from historical memos on "Why it was done this way" to answers to commonly asked operational questions.