Thanks to the February 20th PS4 conference, we know that Gaikai will play a large role in the device and what that role will be. Gaikai will initially be used to allow a player to stream his friend's game in real time, giving that player the ability to comment on the game his friend is playing or even taking over the game for his friend, perhaps to help him advance past a hard part.
In essence, everything involving the "Share" button is all thanks to the power of Gaikai.
Source: http://www.ps4playstation4.com/Expected-PS4-Specs#ps4-news-march-13-2013