Great Spiders are a race of huge, oversized spiders that appear in the Middle-earth legendarium. They appeared in The Silmarillion, The Hobbit, and The Lord of the Rings, all by J.R.R. Tolkien. The film adaptations they have appeared in are The Hobbit: An Unexpected Journey, The Hobbit: The Desolation of Smaug, and The Lord of the Rings: The Return of the King, which were made by Peter Jackson. They seem to despise light, so therefore, they live in dark and perilous places where light can't get in.
The Great Spiders are able to speak to one another, which Bilbo Baggins heard when he put the One Ring on. They are able to spin giant webs, which Frodo got stuck in when he was being attacked by Shelob. They are even able to devour light, which Ungoliant did with Melkor when they destroyed the Two Trees. Ungoliant was even able to grow twice her normal size when she devoured the light from the Two Trees.
Giant Spiders are one of the most common species of Great Spiders in Middle-earth. They lived in the South of Middle-earth at the First Age, but at the Third Age, they spread to other places, including Greenwood, which became Mirkwood after they infested in it. These are also the same spiders that Thorin Oakenshield and the Company faced on their quest to reclaim the Lonely Mountain from Smaug.
These spiders were actually fallen Maiar in the form of spiders after joining Morgoth and his alliances. One example is Ungoliant, who is infamous for teaming up with Morgoth in destroying the Two Trees.
Spawn of Ungoliant
These are the third race of Great Spiders, half-Umaiar spiders, and half-mortal spiders. Therefore, they are immortal, and cannot die. They were spawned after Ungoliant fled to the South, who mated with the mortal spiders already living there.