The ruler of the Eagles in the Third Age of the Sun, his name means 'Wind-Lord' in Sindarin. Gwaihir was a friend of Gandalf after the Wizard had healed him of a wound.
The Eagle and his followers helped Gandalf with several of his tasks. Those we have records of include rescuing Gandalf and Bilbo and the Dwarves from the Wargs and Goblins on their quest to the Lonely Mountain. Later Gwaihir carried Gandalf away from Orthanc, and later still, from the aftermath of his fight with the Balrog. It was Gwaihir and two of his relatives who rescued Frodo and Sam from the lava of Mount Doom.
Gwaihir also led the Eagles in the Battle of the Five Armies against the Orcs.