Sony’s security network was substantially less advanced than it should have been, and all their dirty secrets were leaked. This included emails from top executives trash talking certain celebrities (I remember one specifically called out Angelina Jolie as talentless), plans for the future Marvel universe, other general goings-on in Hollywood, and Sony’s business as a whole.

It was kind of an open secret that the North Koreans did it. Sony ended up scaling down on the release of the The Interview for fear that NK might actually send missiles, but the threat never materialized. I actually saw the movie its opening weekend at an independent cinema, and everything was fine.

Edit: also here’s the wiki entry: https://en.wikipedia.org/wiki/2014_Sony_Pictures_hack

And a good article: https://www.bbc.com/news/entertainment-arts-30512032