Facebook Markup Language (FBML) enables you to build full Facebook Platform applications that deeply integrate into a user's Facebook experience. You can hook into several Facebook integration points, including the profile, profile actions, Facebook canvas, News Feed and Mini-Feed.
FBML is an evolved subset of HTML with some elements removed, and others which have been added that are specific to Facebook. You set the FBML for a profile box by calling profile.setFBML through the API. The FBML is cached on Facebook's server until profile.setFBML is called again through a canvas page. For a high-level technical spec for FBML, please see FBMLspec.
To see which HTML and FBML tags you can use in various parts of your application, such as canvas pages, email, or Feed stories, see Allowed FBML and HTML Tags.