Short answers, pulled from the story.
Legolas is a Sindar Elf of the Woodland Realm and the son of Thranduil, King of Northern Mirkwood. He joins the Fellowship of the Ring at the Council of Elrond in Rivendell, originally arriving as a messenger to report Gollum's escape from his father's guard. He is one of the nine members chosen to carry out the mission to destroy the One Ring.
Orlando Bloom portrayed Legolas in Peter Jackson's Lord of the Rings trilogy, released from 2001 to 2003. Bloom reprised the role in The Hobbit: The Desolation of Smaug (2013) and The Hobbit: The Battle of the Five Armies (2014), though Legolas does not appear in Tolkien's original Hobbit novel.
Legolas and Gimli begin as rivals due to the ancient hostility between Elves and Dwarves, made personal by Thranduil's imprisonment of Gimli's father Glóin. Their friendship develops after Gimli respectfully greets Galadriel in Lothlórien. Tolkien scholar Christina Scull calls it "the greatest reconciliation theme in the book," and the Red Book describes their bond as "greater than any that has been between Elf and Dwarf."
The Sea-longing is a latent desire among Elves to sail west to Valinor, the Blessed Realm. Galadriel warned Legolas in a verse delivered by Gandalf that hearing seagulls would awaken it in him. It activated when Legolas first heard gulls on the coast at Pelargir, and after Aragorn's death he built a ship and sailed west, reportedly taking Gimli with him.
The name Legolas Greenleaf first appeared around 1917 in "The Fall of Gondolin," one of the early stories later collected as the Lost Tales. That character appears only once in the text, guiding survivors out of the sacked city of Gondolin, and is a far smaller figure than the Legolas of The Lord of the Rings.
Tolkien scholar Tom Shippey identified a line Legolas speaks about the hall of Meduseld in Rohan as a direct translation of a phrase from the Old English poem Beowulf. Medievalists Stuart D. Lee and Elizabeth Solopova noted that Legolas's lament over the stones of Hollin echoes the Old English poem The Ruin. Both connections reflect Tolkien embedding his expertise in medieval languages directly into his fictional elf's dialogue.