Social Policies are not new to Civilization V, but they've undergone a heavy revision since the beginning of Gods & Kings. There are two entirely new Branches in the Policy Tree, and three of the existing Branches have been grouped together into a new page with many more options.
Instead of covering these earlier, with the other changes to the game, this major revision has been given its own section. The difference in gameplay is quite substantial, especially in the mid to late game, and you're going to find yourself facing new challenges if you want to master the system.