From Facebook Developer Wiki

Facebook Platform comprises a number of core components, including:

• The API, which consists of a number of methods that let you add social context to your applications.
• FBML, Facebook Markup Language, which allows you to integrate your application into Facebook.
• XFBML, an extension to FBML, which lets you incorporate FBML into Facebook Connect sites and IFrame applications.
• FQL, Facebook Query Language, which lets you quickly and easily query for Facebook user data without using the API.
• FBJS, Facebook's method for letting developers incorporate JavaScript into their applications.