We all know what happens; Chell deactivates GLaDOS, gets put in extended relaxation only to be reawakened years later to fight again and escape. But what really happened in Aperture Science during the intervening years? Have you ever wondered: Why does Aperture look so different in Portal 2?
Most popular community and official game content for the past week.  (?)
What does popular mean?
It means the content you're seeing has been rated up more than it has been rated down for the given time period.