Sindarin is the Elvish language most commonly spoken in The Lord of the Rings films.
In Tolkien's mythology, Sindarin is the language of the Grey-elves and is generally the everyday Elvish tongue heard in the western parts of Middle-earth during the Third Age. It appears in character names, place names, songs and dialogue, giving the films' Elves a language that feels both ancient and practical.
Sindarin and Quenya are related but distinct languages. Quenya was associated especially with the high Elves and had a more ceremonial or literary role, while Sindarin became more widely used in Middle-earth. This is why viewers often hear Sindarin in scenes involving characters such as Legolas, Elrond and Galadriel, although individual characters may use more than one Elvish language.
Tolkien based much of Sindarin's sound and structure on Welsh. The film dialogue was developed and expanded with help from linguist David Salo, who worked from Tolkien's surviving linguistic material. A common mistake is treating every Elvish word in the films as Quenya; the two languages are not interchangeable.