Established in 19 BBY by Supreme Chancellor Palpatine -- who formally reorganized the Old Republic to thunderous applause in the Galactic Senate and assumed the new title of Emperor -- the Galactic Empire is the dominant political entity in the Star Wars galaxy.
The Galactic Empire is a sort of military dictatorship and absolute monarchy combined into one, established when Chancellor Palpatine gave himself the title of Emperor. The Emperor is absolute ruler of the Empire, with supreme power over the government, military, and economy. The Emperor is advised by a sort of Imperial Inner Circle, consisting of his loyal secetaries and advisors. All the Stormtroopers are loyal to the Emperor alone, and execute his will by means of military force.
Until 0 BBY, the Imperial Senate was a major part of the bureaucracy of the Empire, passing some laws and setting the budget. However, Emperor Palpatine dissolved the misguided council when it was revealed the Rebellion held certain Senate seats. The dissolution of the Senate removed the last limits on the Emperor's legislative power.
The Empire incorporates a diverse array of species and cultures, but humans hold all positions of importance under Palpatine's reign. Imperial technology is essentially stagnant, but they already possess technology enabling real-time interstellar communications, trans-galactic travel in a matter of hours, advanced weapons, shields capable of protecting entire planets, and obscene power resources. The most notable innovation during Palpatine's reign is the Death Star, an unprecedented up-scaling of turbolaser technology capable of obliterating an entire planet in an instant.
A small minority of sentient beings in the Empire are able to manipulate an energy field called the Force, enabling them to perform an array of paranormal feats such as telekinesis, telepathy, clairvoyance, and precognition with varying levels of ability.
The Empire's territory spans some twelve million member and conquered worlds, as well some fifty million colonies, governorships, protectorates, and puppet states, spread throughout the entire Star Wars galaxy, from the borders of the Deep Core to Wild Space. 
The Empire is an enormous threat to other civilizations. To prevent unrest, the nature of the totalitarian police state requires an external threat to justify its massive military expenditures. The Empire is threatened from within, however, by the Rebel Alliance, an insurgent group dedicated to the overthrow of Palpatine and restoration of a representative government.
The Galactic Empire is a frequent competitor in versus debates against many other science fiction civilizations, particularly the United Federation of Planets. The Stardestroyer.net site was built as a debate resource for the Empire vs Federation conflict. The Empire is widely regarded as the clear winner against most other science fiction factions.
- Legend of the Galactic Heroes: Which has a nation with the same name.