The Death Star is a fictional moon-sized space station and superweapon appearing in the Star Wars movies and expanded universe. It is capable of destroying a planet with a single destructive super charged energy beam.
The original Death Star first appears in A New Hope. Commanded by Grand Moff Tarkin (Peter Cushing), it is the Galactic Empire's "ultimate weapon", a space station capable of destroying a planet with one shot of it's superlaser. The film opens with Leia Organa (Carrie Fisher) transporting the station's schematics to the Rebel Alliance to aid them in destroying the Death Star. Tarkin orders the Death Star to destroy Leia's home world of Alderaan in an attempt to pressure her into giving him the location of the secret Rebel base; she gives them a (false) location, but Tarkin has the planet destroyed anyway. Later, Luke Skywalker (Mark Hamill) boards the station with his friends and rescues her; in the film's climactic battle scene, he uses his power in the Force to help him destroy the station.
In Return of the Jedi, set four years later, the Empire is building a new Death Star; it is half-finished throughout the film. Emperor Palpatine (Ian McDiarmid) and Darth Vader (David Prowse/James Earl Jones) send the Rebels false information that the station's weapons systems are not operational in order to lure them into a trap, and bring Skywalker on board to turn him to the dark side of the Force. In the film's climax, Vader throws Palpatine down the station's reactor core, killing him, and is mortally wounded in the process. Skywalker escapes with Vader's body moments before the Rebels destroy the station.
Though here at journey's end I lie in darkness buried deep, beyond all towerrs strong and high, beyond all mountains steep, above all shadows rides the Sun, and Stars forever dwell: I will not say the Day is done, nor bid the Stars farewell.