Short answers, pulled from the story.
Namárië is written in Quenya, one of J. R. R. Tolkien's constructed languages. Linguist Helge Fauskanger identifies the variant as "Late Exilic" or "Third Age" Quenya, the form Galadriel would have spoken in Middle-earth.
Namárië is a compressed form of á na márië, meaning literally "be well". Tolkien used this formula for both greeting and farewell in Elvish.
Donald Swann set Namárië to music with Tolkien's direct involvement, published in their 1967 book The Road Goes Ever On. Tolkien rejected Swann's initial proposal and instead hummed a Gregorian chant, which Swann adopted for the setting in the key of A major.
Howard Shore used part of Namárië in the scene titled "The Fighting Uruk-hai" in Peter Jackson's film trilogy, composed between 2000 and 2004. A female chorus sings the poem in Quenya over images of Galadriel watching the Fellowship leave Lothlórien.
Led Zeppelin adapted the first line of Namárië for the opening of "Ramble On", released on Led Zeppelin II in 1969. Tolkien's line "Ah! Like gold fall the leaves in the wind" became "Leaves are falling all around", and the rest of the song references Gollum and Mordor.
Namárië is the longest Quenya text in The Lord of the Rings and one of the longest continuous Quenya texts Tolkien ever wrote. It has attracted the attention of linguists, with Helge Fauskanger producing a word-by-word analysis of the poem.