The Galactic Empire, often referred too as the Empire, is a major political and military organization in Star Wars franchise, ruling the whole Galaxy and fighting the Civil War against the Rebel Alliance (as seen in Star Wars Episodes IV-VI). It is the major enemy power in Dark Forces video game, as well as in the next installments - Dark Forces II and Mysteries of the Sith. It is succeeded by the Imperial Remnant in Jedi Outcast and Jedi Academy.
The Galactic Empire, sometimes referred to as the New Order, was a large organization, covering almost all of the star systems of the known Galaxy. It was established due to a very complicated plot created by Emperor Palpatine, who used his position as a major political figure in the Old Republic to destabilize the Galaxy and destroy his main enemies – the Jedi Order. He rose to power as a Senator from the planet of Naboo, when it was occupied by the Trade Federation – one of the first steps in shaping the future Empire. After being elected as the Chancellor, he used his secret ties as a Sith lord to initiate the separatist crisis, which eventually led to a full-scale war, known as the Clone Wars.
This conflict, which divided the Galaxy into two sides, was used by Palpatine to weaken the position of the Jedi and the Republic, as well as remove all other major powers in the Galaxy, such as the Trade Federation and Techno Union. All of them worked as pawns in a game of dejarik, when one day Darth Sidious had the separatists removed, the Jedi deemed traitors to the Republic, and the Senate accepting the reformation of the Republic into the first Galactic Empire.
For almost two decades the Empire ruled the Galaxy with an iron fist, until a group of freedom fighters, which formed the Rebel alliance to restore the Republic, stole the secret plans from the Empire with the help of Kyle Katarn and later destroyed the superweapon Death Star, capable of wiping out whole planets. Another project from the Imperial Clone Wars veteran general Rom Mohc - the Dark Trooper - was also shut down by Katarn in the years following the battle of Yavin IV.
The conflict between the Empire and Rebels continued for several more years until the Emperor himself perished, when his most loyal ally - Darth Vader - threw him into the shaft of the second Death Star above the Endor moon. This was a major victory for the Alliance, after which the Empire left many of its former star systems, with many of its followers fleeing from justice from the Republic and the whole Galaxy.
After Battle of Endor, the Empire continued to exist as several smaller groups. Each of these was usually led by an individual with ambitions those of an Emperor - this was the case with Dark Jedi Jerec, who sought the power of the Valley of the Jedi in order to proclaim himself the new ruler of the Empire and end the new age of freedom. He and his minions were stopped by Kyle Katarn, who began his training as Jedi at the time. Many other fractions, often called the Imperial Remnant continued to collaborate with various Dark Jedi, such as Desann, who used stormtroopers for his operations along the Reborn warriors and shadowtroopers.
In JO/JA series
Stromtroopers have always been the main enemy in most of the Star Wars stories Aside from JK series, the Galactic Empire is also present in other games, such as Shadows of the Empire and The Force Unleashed. Even though Jedi Outcast places stormtroopers into the category of the Imperial Remnant, they are still called the Empire, at least by most of the Players and fans.
-  - Galactic Empire on Wookiepedia.