Facebook Markup Language (FBML) enabled one to build Facebook applications that deeply integrate into a user's Facebook experience. Deprecated as of June 6, 2012. Not to be confused with xfbml.
Facebook Markup Language (FBML) is a way to tell facebook servers how to interpret certain content and convert this markup to html. It is an evolved subset of HTML from which Facebook has removed some elements, and added others which are specific to Facebook.
Deprecation
Facebook has decided to move away from FBML and towards IFrames while keeping a small subset of FBML available for social plugins and the JS SDK. As of June 6 2012, FBML will no longer function on applications.