The Demonweb Pits were the 66th layer of the chaotic evil-aligned plane of existence known as the Infinite Layers of the Abyss, but are now a separate plane of existence, as of 1374 DR, due to events during the Silence of Lolth.
The layer is the home of evil drow deity Lolth, the Demon Queen of Spiders and many other deities of the drow pantheon of gods. The plane resembles a giant spiderweb across space, formed from strands of interwoven planar matter. Lolth's palace is a giant iron stronghold stylized in the shape of a spider, that crawls across the web strands of the plane.
The 1980 1st-Edition AD&D module Queen of the Demonweb Pits describes the Demonweb Pits as an environment quite unlike the Prime Material Plane, stating that many magical spells and effects do not function as expected here. The web strands themselves are reported to be composed from the lost souls of the Abyss, surrounded by a grey foul-smelling fog above and to the sides of each strand, forming a tunnel-like structure.