For questions about "Star Wars: The Old Republic", a massively-multiplayer online role-playing game (MMORPG) set in the Star Wars universe. Star Wars: The Old Republic (or SWTOR) takes place 4000 years before the events of A New Hope, and is a sequel to the Knights of the Old Republic single-player games. Tag used for questions related to both works. Always use in conjunction with the [star-wars] tag.
The Old Republic, while specifically the subtitle of the MMO game, can also refer to the Star Wars era and setting that it shares with the Knights of the Old Republic video games. Bioware chose this distant past period in Star Wars history (4,000 years before the movies, to be specific) for the creative freedom and stories it would give them while developing Knights of the Old Republic, while still retaining much of the look and feel of Star Wars. Using the events (and popularity) of Knights gave them a starting point to develop the plot and setting further for the MMO.
The initial Knights plot starts and revolves around a single former Jedi named Revan. Revan along with many other Jedi joined the fight against the Mandalorians, who were invading world after world and seemingly could not be stopped by the Republic military alone. After a cataclysmic defeat shatters their home world, the Mandalorians were stopped. Revan and his followers disappeared afterwards, only to resurface claiming the titles of Sith and leading their own armada to take over the Republic. Revan later claims this was for the greater good, seeking to unify and harden the Republic against a threat Revan found in deep space, and fueling his war machine with ancient relics from a long dead Infinite Empire that was able to fuse technology with the Force. Revan's apprentice betrays him, leaves him for dead, and turns his campaign of conquest into one of indiscriminate destruction. Revan survives and is instrumental in bringing down his apprentice and his own army, after which he again vanishes to parts unknown.
Revan's warnings become truth to set the stage for The Old Republic MMO. The true Sith Empire has been in hiding for some time, finding a homeworld on the Outer Rim and building their strength to take revenge on the Republic and the Jedi for their crushing defeat in times past. Having rebuilt their strength, they launch a savage blitz attack, taking over many Republic worlds. When it seems the tide will turn against them, the Republic capital Coruscant is sacked in a daring raid, the Jedi Temple destroyed, and the planet held hostage to force a cease fire. Players are thrust into the resulting cold war, with the Republic and Empire plotting against one another, the Jedi trying to rebuild and rediscover their roots, Sith Lords trying to outmaneuver other Sith Lords, and criminal syndicates on the rise as neutral planets try to stay out of the line of fire. All the while, the shadowy Sith Emperor seems to have his own plans in mind, with dire consequences for all involved...
The setting is well known for blurring the traditional lines between good and evil in the Star Wars universe. Any character can find themselves drawn to the Light or Dark side of the Force, regardless of which side of the conflict they are on. Jedi complacency and Republic bureaucracy and corruption tends to cause just as many problems as Sith megalomania, the Empire's inherent racism, and the constant betrayals and infighting. Heroism and a just cause can be found in the midst of the Empire, while there is plenty of darkness to be fought within the Republic, even before getting into the fight between the two.
This tag should be used for any questions about the story, setting, and characters of any work in The Old Republic setting of the Star Wars universe, including the MMO, the Knights games, and related works. Any questions about game mechanics, or game rules, would be off-topic and should be asked at gaming.stackexchange.com. Questions that fall outside this era should consider star-wars or its many related tags. For more information, please see Wikipedia, TvTropes, the website for the MMO, or Wookieepedia.