Fractals of the mist is a brand new instance coming to Guild Wars which is made up of mini dungeons. As the name suggests it is parts of the mists.
Best to think of this as a massive caverns of time with random locations for each mini dungeon.
Each time you run you randomly get put into 3 diffrent shards of the mists. When you complete all 3 you retrun to face a random boss. The diffuculty slowly incresses the more you make progress and the dungeon is open to all character levels making it very accessable.
Here is the trailer for the dungeon: