The Death Star I was a Giant space station that was built by the Empire. The idea of a "Great weapon" belonged to separatists (Poggle the Lesser and Count Dooku). It's primary weapon, the superlaser, could destroy an entire planet with just one shot. The Death Star was used to destroy the planet of Alderaan in 0 BBY. The Rebels attacked and destroyed the space station during the Battle of Yavin, which occurred shortly after the Destruction of Alderaan. They exploited the stations only known weakness, an exhaust port, by firing two proton torpedoes in to it, which caused a chain of explosions that destroyed the Death Star.
Luke Skywalker in his X-wing, who fired the proton torpedoes, and Han Solo in the Millennium Falcon, provided assistance. They both were rewarded by the rest of the Rebels in the aftermath of the destruction of the Death Star.