Microsoft Active Directory Federation Service is an identity federation technology that is compliant with industry standards such as WS-* and SAML 2.0. ADFS allows an organizations to use claims based access to web applications/services and provide single sign on (SSO) access to web applications. As ADFS is based on industry standards, interoperability with 3rd party federation technology is possible.
The initial version (ADFS 1.0) was shipped from Windows Server 2003 R2 onwards and is available in-box as of Windows Server 2008 R2. The latest version (AD FS 2.0) however is an out-of-band release that can be downloaded from http://www.microsoft.com.
More details are available from http://www.microsoft.com/windowsserver2008/en/us/ad-fs-2-overview.aspx
Claims based identity and access is explained at http://msdn.microsoft.com/en-us/library/ff423674.aspx
IT pro specific content is available from http://social.technet.microsoft.com/wiki/contents/articles/2735.aspx