The Galactic Empire is a galaxy-wide autocratic government ruled by Emperor Sheev Palpatine and the Imperial Senate.
It was formed 19 years before the battle of Yavin at the end of the so called Clone Wars, which was a war between the former Galactic Republic and the Separatist Alliance. Alas the two factions were both serving, then, Senator Sheev Palpatine of Naboo's schemes.
The Galactic Empire was formed for, quoting Sheev Palpatine; "ensuring security and continuing stability", arguing it to be for a, quote, "safer, and securer society". Although, this could not have happened if not Naboo representative Jar Jar Binks had put the action of granting Palpatine the Emergency Powers Act for vote.
The Senate agreed. And shortly after, Palpatine announced the reorganisation of the Galactic Republic, into the Galactic Empire.
However, formality ruled that the Senate itself remain. Limiting Palpatine's powers, even just slightly.
The Empire was ruthless and worked to gain order and efficiency, rather than freedom and thriving. Although this was effective at first, it caused people to grow afraid of the Empire, which later led to hatred. Hatred that formed rebel factions all around the many worlds under the iron fist of the Empire. This later resulted in the creation of the Rebel Alliance. A group of freedom-fighting terrorists, aiming to bring the Empire to ruin.