Generally a 10x10x10 foot square of a muddy brown porous color that floats in the air. When touched, it aligns itself with the soul of the toucher and disappears.
This allows the owner to have a space in another dimension in which he can store gear. He merely has to toss the item in the air and think about putting it in NoWhere and it goes. Bringing the piece back merely is picturing it's return.