One of the primary design purposes for the Ascended item tier is to bridge the gap between Exotic gear (which fairly casual players can have a reasonable certainty of acquiring without too much angst) and Legendaries (which are for the hardest of hardcores). Specifically, Ascended items will be used specifically to combat new endgame mechanics. Foes within the new Fractals of the Mists dungeon will inflict Agony on players. Agony can only be mitigated by Infusions, which can be put only into Ascended armor. As players get to higher levels of difficulty within the dungeon, they will eventually find a point at which they have to counter Agony (by Infusions) before they can hope to proceed. Rather than being upgraded with normal components, Ascended gear has upgrades built-in, and upgrade slots have been repurposed into Infusion slots.
November's update will include only Ascended rings and back items. More Infusions and Ascended item types will be added with time. When Ascended items are added to the game, Legendary weapons will be updated so they are still the best available option (as opposed to being on-par with Exotic weapons, as they currently are).