The Romulan Star Empire is one of the most powerful empires surrounding the Federation, with sizable influence in the Alpha Quadrant and Beta Quadrant and is made up of Romulans, Remans, some Hirogen and an unknown number of subject races with lesser status.
It features as the third playable faction of Star Trek Online, added with the Legacy of Romulus upgrade in May 2013.
2,000 years ago, the Romulans were part of the Vulcan culture, of the same kind and still share a nearly identical biology, but the Romulans left Vulcan after the time of the Awakening, when Vulcans embraced logic, peace, and control of one's emotions. While most Vulcans followed the teachings of Surak, which did not allow the expression of emotions. The Romulans were the minority of Vulcans that refused to give up their passions. They valued love, honor, and war. (Though after leaving Vulcan, encoutering other less honorable races and experiancing centuries of their own internal politcal maneuvering the Romulans became more duplicitous and expedient.)
And so the Romulans left Vulcan and traveled across the void of space in search of a new homeworld, until they found a habitable planet to call home which they called Romulus. Romulus had a moon/satellite slightly smaller, called Remus, in which they met a species called the Remans; the ambitious and xenophobic Romulans enslaved most of the Remans, making them perform slave labour.
The Romulan Star Empire was then founded, expanded in all directions around Romulus, and later introduced to the larger galactic community. The Vulcans didn't even recognize Romulans were their long vanished clan, although they later discovered this. The Romulan Star Empire has had a substantial influence within the Alpha Quadrant although most of the territory of the Empire is situated in the Beta Quadrant. The United Federation of Planets originally came about through an alliance of mutual protection against the Romulans.
The Star Empire was traditionally ruled by a Praetor, and supported by a senate with Senators representing each region of the Empire. Resembling the Roman Empire before the installation of the Emperors. (Although the Romulans have had Emperors and Empresses at times.) Though Romulan foreign policy often sees them prefer to manipulate their enemies from afar rather than commit to open warfare, they have a powerful and technologically advanced military on par with the Federation and Klingon Empire and their joining the allies during the Dominion War was one of the deciding factors in the outcome of the conflict.
The Romulans are one of the few races in the Galaxy known to have invented cloaking technology (sometime during or just before the 22nd Century) which they use extensively in their military. During the early 23rd Century they sold the technology to the Klingons during a short-lived alliance however Romulan cloaks remain slightly more advanced. During the late late 23rd - mid 24th Century the Romulans entered a self-imposed isolation and emerged as a much more powerful Empire, apparently the alliance between the Klingons and Federation was the only thing able to stop them.
Before the Dominion War, the Romulan Tal'Shiar intelligence agency was among the most ruthless and efficient spy organizations in the Alpha and Beta Quadrants. (Alongside the Cardassian's Obsidian Order.) It was said that no Romulan can sit down to a meal without it being noted by the Tal'Shiar. However when the rival Intelligence services of the Romulan Tal'Shiar and the Cardassian Obsidian Order came together to oppose the Dominion, they were manipulated by Changling infiltrators and almost completely destroyed when the Founders manipulated them into making a doomed assault on the supposedly undefended homeworld of the Founders. The Founders had prepared a massive massive Jem'Hadar fleet which wiped out the Tal Shiar and Obsidian Order fleets along with most of the two organizations leadership.
After the Dominion War, the Romulans came to accept the Remans as equals within the Empire after a human clone of Jean Luc Picard, Shinzon, was raised by Remans working in the Dilithium mines of Remus. Once grown Shinzon managed to organise a Reman resistance and built a prototype warship called the Scimitar with a weapon of mass destruction based on thalaron radiation. He also used a miniaturized version to asassinate the Romulan Senate - becoming Praetor for a short while. Shinzon and the Scimitar were destroyed after a battle with the Enterprise-E and it's crew - however the Remans were still released from their slavery.
After a nearby supernova devastated the Romulan star system, the power of the Empire was greatly reduced, with the Romulans even requesting Federation aid. This also caused a Romulan mining captain called Nero - who blamed Spock for not stopping the supernova in time to save his home - to go back in time and create an alternate timeline diverging in the 23rd century. In 2409, the Romulan Star Empire is currently divided between various military commanders and extremely politically unstable. The most notable political rivals are Empress Sela who has the support of the Tal Shiar Intelligence forces and D'Tan who has the support of the Romulan Dissident Movement that support the establishment of a democratic republic. This internal instability within the Romulan Star Empire, along with the weakening of the Cardassians after the Dominion War, leaves the Federation and the Klingon Empire as the two remaining large powers in the Alpha and Beta Quadrants.