Short answers, pulled from the story.
Glorfindel is a High Elf of the Noldor in J. R. R. Tolkien's legendarium, depicted as an Elf-lord of extraordinary power capable of withstanding the Nazgûl. In The Fellowship of the Ring, he assists Frodo Baggins in escaping from the servants of Sauron and participates in the Council of Elrond. Gandalf describes him as "one of the mighty of the Firstborn" and "an Elf-lord of a house of princes."
Glorfindel died fighting a Balrog during the flight from the fallen Elven city of Gondolin in Middle-earth's First Age. He succeeded in killing the creature but was mortally wounded in the process. The eagle Thorondor recovered his body, which was buried beneath a stone cairn where yellow flowers later grew.
Tolkien originally created two separate characters sharing the same name, a reuse that Christopher Tolkien confirmed was unintentional. In a 1972 essay, Tolkien worked out how each Elf's name should be unique and explained how the Glorfindel who died in the First Age could be the same individual who appears in The Lord of the Rings, returned through reincarnation. The History of Middle-earth, completed in 1996, contains the most detailed account of this resolution.
At the Battle of Fornost, Glorfindel prophesied that the Witch-king would fall in the far future, but not by "the hand of man." The prophecy was fulfilled during the Battle of Pelennor Fields, when the woman Éowyn slew the Witch-king, assisted by the hobbit Meriadoc Brandybuck.
Peter Jackson's 2001 film The Lord of the Rings: The Fellowship of the Ring transfers Glorfindel's role to the elf Arwen, who carries Frodo to the Ford of Bruinen and summons the floodwaters through an incantation. In Ralph Bakshi's 1978 animated version of the same story, the role and lines were given instead to Legolas.
Tolkien created Glorfindel around 1916 to 1917, in the earliest drafts of The Fall of Gondolin, which was the first part of The Book of Lost Tales to be written. He read the story aloud to the Exeter College Essay Club in the spring of 1920. The character's name, meaning "blond" or "golden-haired," was among the first devised for what would become the Middle-earth legendarium.