The Rebel Alliance to Restore the Republic, also known simply as the Rebel Alliance or the Rebellion, was a military alliance of planetary, system, and sector-level insurrectionist factions. They were generally united in one common cause: fighting against and defeating a common enemy in the form of the Galactic Empire, in order to restore the former Galactic Republic. To achieve their primary goal in restoring the former Galactic Republic, the Alliance aimed to remove Emperor Palpatine from power, and his hegemonic Imperial Order from the galaxy. Members of the Rebellion were known as "Rebels" in general, and more derogatively "Rebel scum" by stormtroopers, Imperial officers and Imperial sympathisers.
The origins of the overall anti-Imperial movement dated back to 19 BBY, shortly before the Galactic Republic's transformation into the Empire. Some of the Alliance's core founders, such as senators Bail Organa and Mon Mothma. While several resistance factions had existed from 19 BBY to 2 BBY, it would not be until Galen Marek, a former apprentice of Sith Lord Darth Vader, sacrificed his life to rescue Organa and Mothma, that these factions officially unified into a single rebellion, thus creating the Rebel Alliance. The Alliance was publicly and formally declared with the Declaration of the Rebel Alliance and the document of the Declaration of Rebellion.
Hence, the Galactic Civil War erupted with the fledgling Alliance challenging the Empire for control of the galaxy. Rather than challenging the Empire directly, the rebels fought a war of attrition, destroying key Imperial targets and liberating numerous worlds. Though vastly outnumbered in manpower, starships, weapons and various other resources, the Rebels themselves possessed a strong will to fight. The first official attack of the rebellion took place in 3 BBY, where Wedge Antilles led Red Squadron against Imperial forces on Wobani, the base of the Empire's most well known labor camps.
The first significant victory the Rebel Alliance achieved was in 0 BBY, when the Empires' ultimate weapon at that time, the battlestation code-named the Death Star, was destroyed during the Battle of Yavin, by soon to be Jedi Grand Master Luke Skywalker. This victory was thanks to the Alliance successfully stealing the plans for the Empire's ultimate weapon.
Over the next four years, the growing Rebellion waged war against the Empire in thousands of systems and on millions of different world, establishing countless rebel bases including one on Hoth, Crait and Celegia. In addition, the Rebel Alliance established another cause; helping the remaining Jedi to survive, including keeping the location of Obi-Wan Kenobi and Luke Skywalker a secret until 0 BBY. However, under the command of Darth Vader, the Imperial forces succeeded in defeating the Alliance on Hoth, forcing the Alliance to scatter their forces in order to survive.
By and large, the Alliance was composed of well-trained soldiers and brilliant commanders, some of whom dedicated their lives to the Rebellion after defecting from the Empire due to the atrocities that were committed by the Imperial regime, the greatest among them being the destruction of Alderaan.
In 4 ABY, roughly four years after the destruction of the first Death Star, Emperor Palpatine and Darth Vader both perished during the Battle of Endor, and although Grand Admiral Thrawn attempted to reclaim power and re-establish the Empire across the Galaxy in 9 ABY, the Rebel Alliance had officially won the Galactic Civil War. A new treaty named the Galactic Concordance was signed by surviving Imperial officers, ending the war once and for all.
In the wake of their victory at Endor, the Rebel Alliance began to re-organized itself into the New Galactic Republic. Ultimately, centuries later, the New Republic split into two governments, the Great Republic, and the Galactic Alliance, a democratic union that united much of the galaxy under a republican form of government, just as the Old Republic had done in its time.