The Forest Moon of Endor is a moon orbiting the planet Endor; confusingly, it has also come to be known as simply "Endor." The moon first appeared in Return of the Jedi, when the Rebels defeated the Empire and destroyed the second Death Star during the Battle of Endor. It's also the setting of several Expanded Universe works, including the Ewok Adventure movies and the Ewoks cartoon series.
Environment & Life
As the name "Forest Moon" suggests, Endor is mostly covered in evergreen forests. It does contain other climates and features, however, including desserts, mountain ranges, and oceans.
The forests on Endor are teeming with life, including five native sentient species: the bear-like Ewoks; their distant relatives, the Duloks; the amphibious Gorphs; the musical Yuzzums; and the rodent-like Teeks. Native forest creatures include horses, boar-wolves, and fairy-like Wisties. Shipwrecks introduced both sentient and non-sentient species to Endor, including Humans and Jawas.
Endor is a largely unknown and unvisited location due to the lack of direct hyperspace routes and the fact that a black hole, Endor Gate, was located nearby. Most of the new life and technology introduced to Endor came from unintentional spaceship crashes.
Endor's out-of-the-way nature made it a good place for secret projects, such as the construction of the second Death Star during the Galactic Civil War. As a result, it was the site of the historic Battle of Endor in 4 ABY. The Ewoks helped turn the tide of the battle by allying with the Rebels in their attempt to destroy the Death Star's shield generator, located on the Forest Moon's surface.
Some have theorized that debris from the destruction of the Death Star fell to Endor's surface, causing an Endor Holocaust that destroyed all life on the Forest Moon. This is Imperial propaganda, however. The Rebels helped clean up leftover debris from the explosion, and, for a short while, used Endor as their base of operations.
Behind the Scenes
The Expanded Universe is inconsistent on the appearance, or even existence, of the planet Endor. The Return of the Jedi novelization (James Kahn, 1983) and the Dark Force Rising Sourcebook (1993) state that the planet was destroyed and that the Forest Moon orbits as a planet in its own right. However, a planet-like object is visible in the Return of the Jedi film and in the Ewok Adventures movies (1984 & 1985), the novels The Truce at Bakura (Kathy Tyers, 1994) and Dark Apprentice (Kevin J. Anderson, 1994) mention the planet Endor, and the video game Star Wars: Battlefront II (2005) shows Endor as a large blue planet. Since the Return of the Jedi novelization contains several other discrepancies with later canon, it seems best to trust the film and the later Expanded Universe sources.