An AchievementDefinition defines an achievement; this includes its name and how it is unlocked. For an individual user's progress in unlocking an achievement, see AchievementProgress.
This is the type of achievement. There are three types of achievement: AchievementType.Simple - unlocked by completion of a single event or objective, AchievementType.Bitfield - unlocked when a number of bits in a bitfield are set, and AchievementType.Count - unlocked when a counter reaches a defined target.