Smaug was the dragon that ruled Erebor before Bilbo’s quest in The Hobbit.
Smaug attacked the Dwarven kingdom under the Lonely Mountain and drove its people into exile. He then occupied Erebor and its treasure hoard, including the Arkenstone, a precious heirloom of the House of Durin. The nearby town of Dale was also destroyed during the attack.
Thorin Oakenshield later leads a company to reclaim the mountain, with Bilbo serving as the burglar. Bilbo’s conversation with Smaug reveals the dragon’s pride and helps the company identify a vulnerable area in his armour-like hide. A thrush carries information about that weakness to Bard of Lake-town.
Bard ultimately kills Smaug with the Black Arrow. Smaug is distinct from Glaurung and Ancalagon, who belong to earlier First Age stories, and from Scatha, another dragon mentioned in Tolkien’s history.