The JavaScript SDK enables you to access all of the features of the Graph API via JavaScript, and it provides a rich set of client-side functionality for authentication and sharing. The JavaScript SDK is also necessary for using the XFBML versions of the Social Plugins. Many functions in the JavaScript SDK require an application ID. You can get an app ID by registering your application.

For example usage, check out Facebook for Websites and the Authentication guide. We also have a JS Test Console which allows you to test and debug common JS SDK methods.

The most efficient way to load the SDK in your site is loading it asynchronously so that it does not block other elements loading of your page:

  window.fbAsyncInit = function() {
    FB.init({appId: 'your app id', status: true, cookie: true,
             xfbml: true});
  };
  (function() {
    var e = document.createElement('script'); e.async = true;
    e.src = document.location.protocol +
      '//connect.facebook.net/en_US/all.js';
    document.getElementById('fb-root').appendChild(e);
  }());

In the example above, the function assigned to window.fbAsyncInit is run as soon as the Facebook JS SDK is loaded. Any code you want to run after loading the SDK should be placed within this function and after the call to FB.init(). For example, this is where you would test the logged in status of the user or subscribe to any Facebook events your application is interested in.

The SDK is open source and is available on GitHub.

• Core Methods
• Event Handling
• XFBML Methods
• Data Access Utilities
• Canvas Methods

Plugin configuration

• Application ID: Application ID from your Facebook Application settings. Follow this link to register an AppID for your site right now http://developers.facebook.com/setup/
• Facebook Language: Specify the language that you want the Facebook JavaScript SDK to use. Default options defaults to your site language. You can set it to users' browser language or choose one from the list

For more information you can follow this link to learn how it works

http://developers.facebook.com/docs/reference/javascript/

(Source facebook)