Claim based identity is the future of identity management. It is simple, powerful and extensible but the most important reason to use it is the fact that it delegates identity management out of the application.
WIF is Microsoft's infrastructure for using Claim Based Identity. (Similar to what is WCF for networking)
Recently an excellent Training Kit was released about WIF and the integration of WIF with AppFabric Azure ACS.
I strongly recommend to download read and learn.