The eagles seemed to have vanished in the Second Age, perhaps to Valinor. They returned to warn the Númenórians of the imminent destruction of Númenor.
In the Third Age the eagles were led by Gwaihir the Windlord, and aided in many battles and helped many people. They saved the Elves, Men, and Dwarves from defeat in the Battle of Five Armies, helped Gandalf escape from Orthanc, and rescued Frodo and Samwise from the slopes of Mount Doom after the destruction of the One.
Eagles were very large, large enough to easily carry a man, and lived very long lives. They may have been immortal. They could speak Westron, and the languages of Men and Elves. They were sometimes called, because of their size and heroic deeds, the Great Eagles. They exist no more, only their smaller relatives now survive.