XFBML
From Facebook Developer Wiki (FbDevWiki.com)
"XFBML" is the name given to FBML found in iframe Facebook applications to distinguish it from FBML found in FBML Facebook applications. The reason was XFBML was originally a subset of FBML. With the latest JavaScript SDK, you more or less have to experiment with each XFBML/FBML to see if it renders.
As the FBML canvas applications (aka non-iframe applications) are being deprecated, there will no longer be a distinction in the future. As such, this wiki lists all the XFBML as FBML.
