Shelob is a Giant Spider-like creature, she appears at the end of the second volume of The Lord of the Rings, where her attack leads to Sam Gamgee's key decision to take over the quest to destroy the One Ring.
Shelob in the books
Shelob is a huge creature in spider form, high in the mountains of Mordor, the last of the spawn of Ungoliant. Shelob spent her early days feeding off elves and men. Later on when elves and men no longer traveled to Mordor on regular occasions, she was fed Orcs by Sauron. Sauron would often feed her captured prisoners that he had no further use for. Even though they could not communicate, they both understood eachother. Shelob was like a pet to Sauron, and it was good because she guarded the pass of Cirith Ungol to prevent any intruders from entering the dark land.(She may be a form of evil spirit, possibly a Maia or with Maiar blood, instead of an actual spider.) Her lair is along the path that Sam Gamgee and Frodo Baggins take while seeking Mount Doom. Apparently she has some sort of understanding with Gollum, whom the Orcs of the Tower of Cirith Ungol call "Her Shelob's Sneak".
She attacks and stings Frodo, who lapses into a death-like coma. Sam manages to defeat her by letting her impale herself upon Frodo's sword while trying to crush him, and she flees to her lair, wounded.
Thinking Frodo dead, Sam takes the One Ring from him and leaves his body behind, but discovers by listening to a party of Orcs that Shelob's venom is not intended to kill its victims but only to render them unconscious and keep their meat fresh.
The name Shelob is derived from "lob", an archaic English word for spider. A variation, "cob" is the derivation of the word "cobweb".
In the movie, Shelob can be seen to have a retractable venomous sting at the rear end between the spinnerets, resembling a wasp's sting. This is very much unlike real spiders which inject venom with their fangs but accurate relative to the novel. Shelob also appears to have a gaping mouth, whereas real spiders can ingest only liquid. In the book, on the other hand, "clusters" of eyes are mentioned, which may suggest compound eyes like those of insects; the Shelob in the movie does not have compound eyes, which is appropriate. A hunting spider of the family Lycosidae, which Shelob most closely resembles, would have two large eyes facing forward, and a few smaller ones almost hidden below. The only spiders that can reasonably be said to have "clustered" eyes (though not true compound eyes) are the daddy long-legs spiders of the family Pholcidae, but Shelob does not resemble these rather spindly and fragile spiders in other aspects of her physique.