Achievements consist in kind of rewards, they are granted automatically when you accomplish some particular actions in the game. Each of them is represented by an icon and has a value of points. Of course, the most challenging it is, the most points it gives.
If most achievements are visible on the list you can easily get in the game, there are other achievements which can only seen on characters who earn these achievements. Some of them are just secret, some other aren't granted anymore. Public achievements are sorted in several categories, we'll try to include secret achievements in these categories. Finally, there are a few unknown ones which are only known by their icon (according to the file containing all of the achievements icons).
In order to make things easier, achievements have been grouped in several “super categories” in the menu. Each group will include several official achievements categories.