FaceTime debuted on the iPhone 4 as a facility for users to make free calls from iPhone to iPhone video, via Wi-Fi network.
Using a desktop application, Mac users can place calls to users with the iPod touch, iPhone 4. Users stay logged in with your Apple ID and specify an email address that acts as a number.
FaceTime advantage compared Skype actually provide comfort and ease of use. Users do not need to receive or make calls to run applications FaceTime.
Hopefully FaceTime will be integrated in Mac OS X Lion because it can unify the various communications applications Apple on the desktop.