The Galactic Empire's ultimate terror weapon, the Death Stars were battle stations several hundred kilometers in diameter and mounting a directed energy weapon capable of completely destroying an Earth-sized planet with a single shot.
Two such Death Stars were featured in the original movie trilogy, the first in A New Hope, and the second in Return Of The Jedi. In the first movie, Luke Skywalker pilots an X-Wing starfighter through a trench-like indentation on the surface of the Death Star, evading a pursuing Darth Vader long enough to launch a proton torpedo down a ventilation tube that reaches right down into the "reactor core", causing a chain reaction to destroy the battle station. In Return of the Jedi, a group of fighters and the Millennium Falcon piloted by Lando Calrissian fly into the centre of the ship through a narrow maze of pipes and destroy the reactor directly, rushing out in just enough time to escape the ensuing explosion.
Outside of the movie trilogy, there was also a prototype in the Maw installation near Kessel that was destroyed by the New Republic. Durga the Hutt also built a small version with only the central laser core and a small living quarters, which was destroyed in the asteroid field around Hoth.