The Infinite Layers of the Abyss is the chaotic evil outer plane of existence, the home of one of the principal combatants in the Blood War, the Tanar'ri. It is shaped as unending layers upon each other, stretching to infinity, though some scholars believe there's a cap of 666 layers. No one can prove this, as no one has been able travel that far (and survive).
The abyss is a wellspring of chaos, ever shifting in destructiveness. The only constant in this chaotic realm is the presence of the adjacent "Blood Rift" where the demons fight their mortal enemies the devils. The proximity of the Blood Rift to both the Abyss and the Nine Hells allows for the demons and devils to wage an unceasing war upon each other. The ever shifting nature of the Abyss brings different layers into proximity with the Blood Rift at different times, allowing the Demons to constantly bring fresh forces to bear in the constant struggle.
Each layer is different from all the others, the only apparent similarity being that the plane seems to try and inflict as much harm on travelers with each layer as possible. It might even be said that the plane is as varied and evil as the demons who inhabit it. The Demonweb Pits used to be located on the 66th layer before being removed as an independent plane.