The Watchers were evil spirits living in the stone statue gate-posts, acting as guardians of the orc tower past Cirith Ungol, beyond Shelob. They could see both the visible and the wraith-world. The only way to pass them was by having will-power greater than their malice, rare indeed. In the Return of the King, they are described as black shapes, great seated figures with glittering eyes. Sam and Frodo passed using the phial of Galadriel and calling upon the Valie Varda, using her elven names of Gilthoniel and Elbereth. That broke the will of the Watchers and caused the keystone of the arch to fall. The Watchers then could only send up a wail that brought a winged Nazgul from which the hobbits hid.
Reference: "The Tower of Cirith Ungol", Return of the King
Image of the Watchers by Howard Koslow, from the Rolozo Tolkien page