Sindarin is the fictional language J.R.R. Tolkien developed for the Sindar, a group of Elves in Middle-earth.
In Tolkien's mythology, Sindarin is associated especially with the Grey-elves of Beleriand. It became the everyday Elven language spoken by many Elves in Middle-earth during the later ages. Tolkien based much of its sound and historical development on Welsh, one of the real languages that strongly influenced his artistic approach to language creation.
Quenya is a related Elvish language associated with the Noldor and with older, formal, or ceremonial usage. The two languages share a fictional ancestry but are not interchangeable. Tolkien developed their vocabulary, grammar, and sound changes as part of a larger historical system.
Khuzdul is the secret language of the Dwarves, while the Black Speech was created for servants of Sauron. These distinctions are frequent sources of confusion because all appear in Tolkien's linguistic world.