The Romulans left the Vulcan home world before the revolution in thinking, urged on by Surak, had overthrown the old militaristic ways. A near-totalitarian military government rules Romulan society, and duty to the Romulan Star Empire is required to be placed above personal safety in all circumstances. All citizens are expected to give their lives for their government without question. They are xenophobic to other races.
Physically and Behaviourally, Romulans and Vulcans are very similar and yet dissimilar.
Physical: Both have pointed ears, giving them excellent hearing, and, like Vulcans, have an inner eyelid that will allow for near-instantaneous adjustments to varying light levels. They also have an extended life span. Romulan ribosomes apparently do not match those of humans or, surprisingly, Vulcans -- but can match those of Klingons. Their metabolism appears to be faster than human standard; the higher rate usually helps towards a faster physical recovery.
Romulans are not quite as strong as Vulcans, because the world they chose to colonise, named Romulus by the Federation, has a slightly lower gravity than Vulcan does.
Behavioural Customs: The race still believes in discarding genetically or physically inferior infants, and that it will conquer the galaxy. They are intelligent, cunning, and very commonly ruthless. They also frequently revert to subterfuge in order to complete goals. They view the Federation as exploitative and shortsighted.
Romulan technology includes the "cloaking device" allowing them stealth and making them formidable opponents.
Throughout their long history of war Romulans have rarely attacked first, opting instead to test their enemies' resolve. A very moral people with an absolute sense of right and wrong, enemy and friend they follow a path called the Men'hsai. Many ordinary Romulans, even those in the military, have horror stories about treatment by the Tal Shi'ar -- a military service which appears to operate above the standard empire. Natives learn quickly to be secretive and not to volunteer information. Women as well as men can climb high in both the military and the Tal Shi'ar.
The Romulans' preference for waiting and observing while other make the first move would explain why they have never ventured into the Gamma Quadrant. The species is made up of many varied skin tones, from a yellow tinged to a dark brown. As a rule, they claim to have invented every technological breakthrough before any other culture.