The Yasawa Islands are a string of roughly 20 volcanic islands stretching northwest from Viti Levu, reachable from Port Denarau by ferry or seaplane. They're best known for the Sawa-i-Lau limestone caves -- a system of flooded caverns you swim into, with a hidden upper chamber lit by a natural skylight -- and for the Blue Lagoon, the turquoise bay made famous by the 1980 film of the same name.
Most visitors base themselves at one of the resorts dotted along the chain and day-trip or island-hop from there, though the caves and lagoon are also commonly visited as a single full-day tour from Denarau. Compared to the Mamanuca Islands closer to Nadi, the Yasawas are quieter and less developed -- the appeal here is remoteness and water clarity rather than resort infrastructure.
Getting there is part of the experience: most transport to the Yasawas runs via Port Denarau, so a transfer to Denarau is usually the first leg of any Yasawa trip from Nadi Airport.