With Windows Phone 8.1, Microsoft is binding its core services even more tightly together to provide the ultimate user experience. Skype, acquired by Microsoft over a year back, now features deeper integration into the OS, as well as is tied to Cortana's voice recognition functions. For example, you can ask Cortana to Skype call one of your contacts, and the video call will be instantly made. Such is the level of integration to be found in Windows Phone 8.1.
The second video details some of the improvements made to Skype in WP8.1.