Shelob is the giant spider defeated by Sam in The Lord of the Rings.
Shelob lives in a lair near Cirith Ungol, a pass leading toward Mordor. Gollum guides Frodo and Sam toward her, hoping that the spider will kill them and leave the One Ring available for him to take. Shelob attacks Frodo, stings him, and appears to have killed him.
Sam rescues the situation by taking the Ring, using the sword Sting, and confronting Shelob with the Phial of Galadriel. He wounds and drives her away. This episode is one of Sam’s clearest acts of courage and loyalty, and it allows him to continue the mission when Frodo is incapacitated.
Ungoliant is an older and more powerful spider-like being from The Silmarillion, and Shelob is described as her descendant. Aragog belongs to the Harry Potter series, while Lobelia Sackville-Baggins is a hobbit, not a monster.