Short answers, pulled from the story.
Shelob is a fictional giant spider in J.R.R. Tolkien's The Lord of the Rings, described as the last child of Ungoliant and as an evil that predates Sauron and the fortress of Barad-dur. Her lair is in Cirith Ungol, the mountain pass leading into Mordor. Sauron leaves her alone as a useful guard on the pass, occasionally feeding prisoners to her.
Samwise Gamgee defeats Shelob by wounding her with the Elvish sword Sting, and the Phial of Galadriel, containing pure light from the Silmarils, nearly blinds her. Seeking to crush Sam, Shelob drove herself onto the upward-pointing blade. Her eventual fate, Tolkien notes, is something "this tale does not tell."
Tolkien explained in a letter to his son that Shelob means "she + lob," with lob being an archaic English word for spider, influenced by Old English loppe. The word lob is not related to "cob" or "cobweb." Tolkien also used the Old English spider-words attercop and cob in The Hobbit, in songs Bilbo Baggins sings to the giant spiders of Mirkwood.
Peter Jackson based Shelob's physical design in The Lord of the Rings: The Return of the King on the New Zealand tunnel-web spider. Jackson has admitted he hates the species.
Scholar Brenda Partridge traced Shelob's literary origins to four sources: the Norse myth of Sigurd killing the dragon Fafnir, the Greek legend of Theseus and the Minotaur, the myth of Arachne, and Milton's Sin in Paradise Lost. George H. Thomson also compared Shelob to Milton's personifications Sin and Death, writing that she represents "the Death and Chaos that would overcome all."
In the video game Middle-earth: Shadow of War, Shelob appears as a major character who shape-shifts into the form of an attractive woman and serves as both narrator and ally to the player character Talion. Creative director Michael de Plater described Gollum and Shelob as "the unsung heroes of The Lord of the Rings" and envisioned Shelob as a dark counterpart to Galadriel, one who manipulates lesser beings but is, in his view, more honest about it.