(covers information from several alternate timelines)
Romulus was the inhabited second planet of the Romulan system in Sector Z-6 of the Beta Quadrant. It was the adopted homeworld of the Romulans following their exodus from Vulcan, and the capital world of the Romulan Star Empire.
During the Neutral Zone Incursion of 2266, the location of the Romulan system was labeled on a map of Sector Z-6 that was displayed on the bridge viewscreen of the USS Enterprise. (TOS: "Balance of Terror", production art)
In 2293, the location of Romulus in the Milky Way Galaxy was labeled in a star chart that was in Captain James T. Kirk's quarters aboard the USS Enterprise-A. (Star Trek VI: The Undiscovered Country, okudagram)
In 2374, the location of Romulus vis-à-vis the Cardassian-Romulan border was shown on a graphic display in a holo-simulation. According to the simulation, Dominion and Cardassian forces would reach the planet within one day of crossing the border via the Glintara sector. (DS9: "In the Pale Moonlight")
- See also: Romulan history
In the 4th century, a group of Vulcans left their homeworld Vulcan and settled the planets Calder II, Dessica II, Draken IV, Yadalla Prime, and Barradas III. This civilization, called the Debrune, was regarded as a predecessor of the Romulan Star Empire. After their odyssey, which lasted for centuries, the Vulcans finally reached and settled Romulus. (TNG: "Gambit, Part I", "Gambit, Part II")
In 2256, Captain Gabriel Lorca pointed out to Michael Burnham that, while experiencing the possibilities of the mycelial network, a traveler might blink and, before they knew it, they had "missed" Romulus. (DIS: "Context Is for Kings")
In an alternate 2258, Christopher Pike was held hostage aboard the Narada after the Destruction of Vulcan, though he tried to assure Nero, who had witnessed Romulus' destruction in 2387, that Romulus, as of then, still existed in that timeline. (Star Trek)
During his time as an Obsidian Order operative, Elim Garak worked at the Cardassian embassy on Romulus as a gardener. He claimed the dominant color there to be grey. (DS9: "Broken Link", "Inter Arma Enim Silent Leges")
In Grathon Tolar's forged holoprogram, a hologram of Weyoun stated that the Founders believed that they would be on Romulus in less than three weeks after a Dominion invasion. (DS9: "In the Pale Moonlight")
During the Dominion War, injured Romulan soldiers were taken to Romulus for treatment, until Senator Kimara Cretak successfully persuaded the Bajorans to let them establish a hospital on Derna. (DS9: "Image in the Sand")
In 2387, Romulus's sun went supernova, endangering the entire galaxy. Ambassador Spock created a red matter singularity which consumed the star, but not before the supernova reached Romulus, fragmenting the planet and annihilating its inhabitants. (Star Trek, PIC: "Remembrance")
In an alternate timeline experienced by Jean-Luc Picard in 2370, Romulus remained intact but had been conquered by the Klingons by 2395. Romulus was cut off from the Federation, except for the occasional medical ship allowed through to help stop an outbreak of Terrellian plague. A new future was created by Picard in 2370. (TNG: "All Good Things...")
Romulus and Remus were the major worlds of the Romulan Star Empire.
Romulus appeared as a temperate planet, which harbored numerous large bodies of water. Although both planets orbited their central star, Romulus and Remus were often referred to as twin planets. Romulus was slightly larger in size.
- Cities and towns
In Roman mythology, Romulus and his brother Remus were known as the brothers who founded the city of Rome. Romulus became the first king after killing his brother. The relation between the planets of Romulus and Remus reflects this.
The Romulans and the Romulan Star Empire were referred to exclusively as an Alpha Quadrant power throughout Star Trek: Deep Space Nine and Star Trek: Voyager, such as in "The Search, Part II" or "Flesh and Blood", and "In the Pale Moonlight" clarified that that the Romulans and Cardassians shared a common border. Despite this, a graphic placing the Romulan Star Empire in the Beta Quadrant made a brief on-screen appearance, as a display graphic on a PADD in Star Trek: Insurrection.   The information on the PADD, though, was completely illegible on screen.  The script of 2009's Star Trek likewise referred to Romulus as being in the Beta Quadrant, in a line of dialogue which is not in the final edit of that film. 
Production Designer Herman Zimmerman commented in Star Trek: The Magazine about his work during Star Trek Nemesis: "The Romulans have possessed advanced technology a lot longer than the Federation, so the look was a combination of art deco and medievalism meets high tech. Most of the designs were inspired by Italian designer Carlo Scarpa." (citation needed • edit)
Regarding exterior design of the Senate area, designer Syd Dutton said, "[Director] Stuart Baird wanted us to think about Albert Speer, the architect who did all the conceptual drawings for Hitler. Speer took that national socialist idea to a ridiculous extreme where everything was huge and classical [....] The Romulans are a people who live in a marshy area. They had little houses on stilts around mudwork. The mudwork became part of this central core and that was where the old part of the city – the Forum and Senate buildings – was located. As the city expanded going away from that, the buildings became bigger and more technological." (citation needed • edit)
In virtually all reference works, Romulus and the Romulan Star Empire were depicted to be located in the Beta Quadrant. These books included such works as Star Trek: Deep Space Nine Technical Manual (p. 2), Star Trek: The Next Generation Technical Journal (p. 9), Star Trek: The Next Generation Writers' Technical Manual (p. 20), Star Trek: Voyager Technical Manual (p. 27), Star Trek: Star Charts (pp. 45, 55, 56, et al.), and Stellar Cartography: The Starfleet Reference Library ("Beta Quadrant: Local Space Extract" and "Federation Historical Highlights, 2161-2385"). Even reference works that date back to a time before the Alpha and Beta Quadrant distinction was invented depicted the Romulans in the area that was later classified as the Beta Quadrant. These included such works as the Star Trek Star Fleet Technical Manual (T0:02:07:00) and Star Trek Maps (Chart A). According to the Star Trek Encyclopedia (3rd ed., p. 43 & 393), the Empire also included space in the Alpha Quadrant. The fact that DS9: "In the Pale Moonlight" shows Romulus as being only one day away from the Cardassian-Romulan border renders the aforementioned reference works inaccurate, since none of them show the Cardassian Union and Romulan Empire with a mutual border.
According to Star Trek Maps (Chart D), the two moons of Romulus were called Megara and Amyclae.
According to Star Trek: Star Charts (pp. 55 & 66), the official name for this planet was Romulus. It was an M-class planet located in the Romulus system and was the capital of the Romulan Star Empire. The planetary capital was also named Romulus. The dominant species were the humanoid Romulans. In 2378, the population of this world was estimated to be eighteen billion. The ancestral homeworld of the colonists, Vulcan, was warp-capable in 320 AD. Approximately fifty years later, these ancestors left Vulcan at the time of the Great Awakening. This planet was located in the Romulan War Theater of Operations (2156-2160).
According to Stellar Cartography: The Starfleet Reference Library (p. 34), the Romulans, who referred to themselves as Rihannsu, called their homeworld ch'Rihan. In their ancient language, which was a derivative of High Vulcan, they called the planet Rom A'losh ("Raptor's Nest").
In her novelization Unification based on the TNG episodes "Unification I" and "Unification II", executive producer Jeri Taylor gave the name Dartha as the capital of Romulus. However, as of the subsequent novel Vulcan's Heart by Josepha Sherman and Susan Shwartz, Pocket Books "officially" standardized Ki Baratan as the name for the Romulan capital.
In the Rihannsu novels, Romulus was called ch'Rihan and Remus was ch'Havran, and the Romulans were the Rihannsu (all of which being their names in the Romulan language). Remus was said to be the more temperate of the two planets. Also, Romulus was said to have at least four moons.
In Star Trek: Federation - The First 150 Years Romulus was called Rom A'losh, which translates as "Raptors Nest".
The comic book series Star Trek: Countdown, a tie-in to the 2009 Star Trek film, gave further detail about the destruction of Romulus. The supernova which destroyed the planet was identified as being in the Hobus system; Nero and his crew were mining in that system at the time of the star's first eruption. Prior to the destruction of Romulus, Spock attempted to warn the Romulan Senate that the supernova posed a threat to both the planet and the entire Romulan Star Empire, but was rebuffed.
In the video game Star Trek Online, the supernova that destroyed Romulus was intentionally caused by a faction of Romulans following the orders of the Iconians. The game's backstory, which included the Countdown comics, also explained the physical impossibility of a star in another system destroying Romulus with no possible evacuation time by stating that the supernova's shock wave propagated through subspace at faster-than-light speeds (implied in game to be the result of the manner by which the nova was induced).