Gaikai was announced to be Sony's cloud-based service for the Playstation 4 when the system was first revealed. They intend to develop a cloud streaming service that will allow people to play Playstation 3 titles, as well as play PS4 titles on the Vita via Remote Play.
It is with Gaikai that players will be able to utilize some of the PS4's sharing functions, as well as social aspects, such as being able to take control of a friend's game during a difficult section. The service to stream titles from previous generations won't be available at launch, but will be made available first to North America sometime in 2014.