For information on the Romulan Star Empire, see Romulan Star Empire.
Romulans (Rihannsu), physically, are almost indistinguishable from Vulcans. Once one gets past the green skin tone and the pointed ears, however, the similarities all but stop. Romulans are descendants of Vulcans, those who rejected the teachings of Surak around two millenia ago and fled Vulcan, riding sublight ships the incredible distance from Vulcan (known as 'the Journey') to their eventual twin homes of Romulus and Remus, known to the Rihannsu as Ch'Rihan and Ch'Havran. But despite this common heritage, the modern Rihannsu shares virtually nothing in terms of their mental and emotional makeup with their distant, more logical cousins.
When the Vulcans promote peace within certain bounds, the Rihannsu likewise promote war. Where the Vulcans look for perfection in logic and cold calculation, Romulans embrace their emotional roots, freely embracing the feelings that the Vulcans shed like a set of clothes long before any current Rihanna or Vulcan's great-grandparents were a twinkle in somebody's eye. The differences in philosophy and culture, combined with millenia of isolation, meant that before the Vulcans and the Romulans rediscovered each other, they had grown so far apart that genetic differences had long since settled in between the two.
While Vulcans, or their parents, grew up under the hot sun of Vulcan and enduring heavy gravity and thin air as a matter of course, the adopted homes of the Rihannsu are far more Earth-like in nature. As a result, the considerable physical gifts with which the Vulcans had been endowed have almost entirely vanished in the Rihannsu. They are more nimbler than a Human, but they share their Vulcan cousins' relatively weak constitution. Their psychic powers, so well-noted in the Vulcans, have left with time, and their prodigious mental powers have also deteriorated.