The great spider that attacks Frodo in Cirith Ungol is Shelob.
Shelob lives in a pass above the road to Cirith Ungol in Mordor. Gollum guides Frodo and Sam toward her lair, intending that she will kill them so he can recover the One Ring. Shelob attacks Frodo, stings him, and leaves him apparently dead, but Sam uses Sting and the Phial of Galadriel to drive her away.
Shelob is described as a descendant of Ungoliant, the enormous spider-like being from Tolkien’s earlier mythology. She is not one of the ordinary giant spiders found in Mirkwood. Sam temporarily takes the Ring after believing Frodo dead, then returns it when he discovers that Orcs have carried Frodo into the Tower of Cirith Ungol.