MAJOR SPOILER, DON'T READ IF YOU DIDN'T FINISH STORY MODE. So, in the end when Ezio basically says that he is ending his reign as Assassin, in a pretty emotional connection he had with Desmond through the power of the Apple. And the credits roll. And, as always, after the credits finish, the game loads, you can continue to free-run, and continue the rebuilding of the city Ezio was helping. But you basically return with him as..an Assassin. I understand that the free-running mode after you beat the story mode is something that people can do for fun, but I've never really continued it, (with II, Brotherhood, and now Revelations), but it just seems a little weird that he'd quit being an assassin, and when the game continues after the credits, you can continue Ezio's life..as an Assassin.
GT: Lilleno57 Left 4 Dead and Gears of War for life.
The post-game stuff is just there so you can do anything you didn't do before you beat the game. It's only post-story in the sense that it's there once you've beaten it. It just becomes a sandbox at that point.