The Galactic Empire existed from 19 years BBY to 9 years ABY.
Darth Sidious was at the centre of events in the guise of Senator Palpatine. His dual identity was a closely guarded secret and as Palpatine he gradually moved the Council towards giving him total control.
As his power grew, he expanded the Imperial forces, declared war on the Jedi Order, and in time, declared himself Emperor.
The Empire became a tyrannical state with absolute power in the form of the Death Star and a fleet of Star Destroyers as well as stormtrooper armies posted on many planets to crush local opposition.
Death Star was destroyed, but a larger, more powerful Death Star began construction shortly afterwards. The Rebel Alliance exploited weaknesses in both cases and caused the destruction of both Death Stars and the overthrow of the Empire.