Better known as the Elvenking, Thranduil was an elf, presumably Sindarin
given his name-form, who ruled a realm in northeastern Mirkwood
in the Third Age of Middle-Earth. He appears in several of Tolkien's novels. In The Hobbit, he is portrayed as greedy and possessive, refusing to free Thorin and his companions until they told him of their quest and later riding against them to claim a share of the spoils from Erebor.
However, he fights with the forces of good at the Battle of the Five Armies, and also later, during the War of the Ring. He captures but fails to hold Gollum. He then sends an elf from his household, Legolas, to Rivendell, and Legolas becomes one of the fellowship of the ring. Thranduil is presumably involved in the battles around Dale, and appears in all the battle-strategy games based on Lord of the Rings, but he never appears in person in the book.
His realm is made up mainly of Laiquendi/Nandor/Sylvan Elves. They are powerful in magic and keep themselves well-hidden, coming out only to feast and hunt by night. His halls are also protected by magic, and are basically a maze of underground caverns and dungeons similar to Gondolin and Nargothrond. The realm, sometimes termed the Woodland Realm or the Elvenking's Realm, does not seem to have a specific elven name.