Zelda Classic quests are custom "games" built with the Zelda Classic Quest Editor. These quests use Zelda Classic as their engine, so the core is the original Legend of Zelda. However, over the years, Zelda Classic has grown, and will continue to grow, allowing new features to be used each time a new version comes out. At this point in time, Zelda Classic quests are so customizable, and have so many features, that it can be hard, or even impossible, to tell that the core engine is the same as the Legend of Zelda. When downloading quests, be sure that you have at least the version of Zelda Classic specified on the download page.