I would look at the Facebook platform as a set of APIs:

Facebook canvas applications (Apps you use in FB. What users think of as "Facebook apps")

FBML / FBJS apps

Iframe canvas apps

Facebook desktop applications (Rare)

Facebook connect applications (Websites with elements of FB in them. CNN, Digg)

Web

iPhone

Note that all of these can access the Facebook API, the REST and FQL interface. Most of the

documentation is for FBML canvas applications. On the left side of the Facebook developer wiki you can

see a few top-level options:

API (you can always use this)

FBML (canvas apps only)

XFBML (Facebook connect only)

FQL (you can always use this)

FBJS (mostly canvas apps, some connect functionality)