So, go there and figure it out.
After creating account you add games to your list, games that you played and finished, games that you are playing right now. It's great for tracking the state of your collection/gamer history.
Then you'll know answer to this question:
Beaten games - Games you finished on any difficulty, e.g. completed main story in Assassin's Creed
Completed games - Games that you finished on highest difficulty with all bonus stuff found/done, e.g. completed Assassin's Creed and found all flags, templars etc.
Mastered games - Completed games that you finished on additional personal challenge, e.g. completed Assassin's Creed without being hurt by guards or on speed run etc.
Null - Games that don't have ending, but you play them anyway, e.g. Chicken Invaders
The best thing about Backloggery is that its your personal list, you're having it for your own sake, so saying you completed something but you actually haven't is lying to yourself, and that is sick.
But be warned, you may become "little obsessed" about getting your games to completed status. I know I have.
