Emperor Palpatine, known also as Senator and Chancellor Palpatine and Darth Sidious, is a significant character throughout five of the first six movies of the Star Wars franchise. In the prequel trilogy, he is not yet an Emperor (he started out as a senator representing the planet Naboo in Episode I: The Phantom Menace then was elected Chancellor of the Galactic Senate via manipulation of the Senate later in that movie), but in the original trilogy, he fulfilled that role, having declared himself Emperor and dissolving the democratic Galactic Republic in favor of forming the totalitarian Galactic Empire in Episode III: Revenge of the Sith.
During his reign as Emperor, Palpatine ruled the Galactic Empire with an iron fist, using his Sith apprentice, fearsome cyborg Darth Vader (the former Jedi knight Anakin Skywalker, who Palpatine turned to the Sith in Episode III) as his main enforcer, along with the Imperial Stormtroopers as his military force and the gigantic Death Star superweapons (the original Death Star in Episode IV: A New Hope and the even larger Death Star II in Episode VI: Return of the Jedi), to impose his will and spread terror upon the Empire's subjugated populations.
In Episode VI, Palpatine met Luke Skywalker for the first time aboard Death Star II and tried to get him to kill him with his lightsaber, using the Battle of Endor (which was raging on the planet of that name below and in its skies above, and which the Imperial forces had taken the upper hand in) to compel Luke to turn to the dark side of the Force and become a Sith as well, leading to a lightsaber duel between Luke and his biological father Vader. When Luke defeated Vader in the duel but refused to kill him, then declared himself a Jedi like his father before him, Palpatine sadistically tried to kill Luke by using Force lightning against him. The mortally wounded Vader stopped Palpatine by seizing him and throwing him down the reactor shaft of Death Star II, killing the Emperor and leading to Vader's redemption and transformation back to Anakin Skywalker, shortly before his own death.
George Lucas, when writing the character in the original trilogy, and to a lesser extent the Prequel trilogy, had been inspired by Richard Nixon, who at the time the film was undergoing development was undergoing the Watergate Scandal. Part of this reason was because he also intended for Star Wars to be anti-War propaganda as well as "a decisive blow against American Imperialism." Lucasfilm also implied that Palpatine also had bits taken from Emperor Constantine, Julius Caesar, Adolf Hitler, and Joseph Stalin.