Ran across this great developer-centric article by Keith Brown that explains the basics of Active Directory Federated Services. It describes how ADFS implements a web site single sign on mechanism that supports cross domain and cross platform scenarios. Something that most developers are running into more and more these days. For a system-admin centric view see Simplify Single Sign-on Using ADFS by Matt Steele.
Here's an ADFS Step-by-Step guide
Keith Brown also wrote an artlce titled "The .NET Developer's Guide to Identity" and sets a bit more context around possible use of ADFS.