Physically, Romulans are almost identical to their cousins the Vulcans.  Externally most Romulans differ from Vulcans in having prominent ridges of bone on their foreheads, 
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a trait which evolved after their departure from Vulcan in circa 369.  However, the Romulans lack the rigorous mental disciplines developed by the followers of Surak 
and this, combined with the more benign environmental conditions on Romulus, has greatly limited the physical capabilities of Romulans compared to their ancestors.
Like the Vulcans the Romulans have surrendered unrestrained violence as a way of life. However, in the case of the Romulans this has been replaced with a controlled deviousness; as a species the Romulans are generally thought of as duplicitous, a reputation the actions of their government over the last decade has reinforced.  Their reluctance to rely on overt hostility generally leads the Romulans to play a waiting game, attempting to manipulate an opponent into breaking - or appearing to break - an agreement so as to give them a solid justification for striking. 
Once the planet Romulus was colonised, the Romulans rapidly spread to their system's second habitable planet, Remus.  From there they founded an interstellar empire spanning many worlds. In the 2150's this empire came into contact with the rapidly expanding Human sphere of influence. First contact came when the NX class USS Enterprise wandered into a Romulan minefield, sustaining significant damage. A pair of Romulan vessels then forced the Starship to leave the area, despite having a mine still attached to the hull. 
Relations between Humans and Romulans continued to be poor, and eventually a brief but intense war resulted. This war was fought entirely in space, without any kind of personal or visual interaction between the two sides; no Human or Vulcan saw a Romulan until 2266, long after the war was ended, when a Romulan Bird of Prey penetrated the Neutral Zone to test Federation defences. Although the ship was destroyed by the USS Enterprise,  this action marked a long period of 'cold war' between the two powers. Eventually this culminated in the Tomed Incident of 2311, when thousands of Federation lives where lost to the Romulans. The incident led to the Treaty of Algeron, which forbade the Federation from developing cloaking technology and reaffirmed the existence of the Neutral Zone between the two powers. Subsequent to this the Romulans cut off all contact with the Federation. 
Although the Starship USS Enterprise-C engaged several Romulan vessels attacking the Khitomer outpost in 2344,  and there were other unofficial contacts, in general the Romulans maintained their isolation until 2364. At this point a Romulan warbird entered Federation space to investigate the loss of several of their border outposts.  Although it later emerged that the Borg where responsible for the destruction,  the Romulans announced at this time that they would again take a more active role in international relations.  They acted in opposition to the Federation for some years,  until the emergence of the Dominion threat in 2370. The Romulans responded by allowing the Federation to utilise one of their cloaking devices on the USS Defiant in order to allow it to operate in the Gamma Quadrant. 
Subsequently the Romulan Tal'Shiar launched a joint attack on the Dominion with the Cardassian Obsidian Order; the attack failed utterly, devastating the Romulan and Cardassian fleets.  After this the Romulan government signed a non-agression pact with the Dominion.  The Romulans reneged on this treaty after a successful intelligence operation conducted by Captain Sisko, joining forces with the Federation-Klingon alliance in the war against the Dominion. 
After the war there was something of a political upheaval on Romulus. Shinzon, a clone of Captain Picard created as part of a Romulan intelligence operation, had been exiled to Remus early in his life. Here he gained a considerable following, becoming a highly respected military leader during the Dominion war. Shinzon assassinated the Romulan senate after the war and plotted to destroy Earth, but the attempt was foiled by Picard and the crew of the Enterprise-E.