Romulus
If Vulcan is a planet shrouded in mystery, Romulus is perhaps even more enigmatic. As with Vulcan, some of the best information available about the history of the Romulans is contained within non-canonical works - most notably Diane Duane's Rihannsu series. Romulans made their first appearance in Balance of Terror, but it was not until The Next Generation's Unification that we see the planet itself for the first time. Little is known about this planet, but we shall attempt to cull together a few sufficient facts.
Climate. Located in the Beta Quadrant, little is really known about the planet Romulus except that it is very much the opposite of Vulcan. Romulus is a temparate Class M planet, with large bodies of water and abundant vegetation. It has been described as humid and has been depicted as looking overcast on the large and small screens.
Biology. Like Vulcans, Romulans are a humanoid species. As they are related in some way, they share many similar chracteristics. In Balance of Terror and The Enterprise Incident, the two species are indistinguishable from one another. It has been suggested (in an article on StarTrek.Com) that Vulcans and Romulans are separate species from the same world, much the same as Homo Neandertalensis and Homo Sapiens co-existed on Earth. One species has a smooth forehead and the other, has v-shaped ridges. There is some debate as to whether these two races truly originated on Vulcan or on Romulus.
History. During the Vulcan Time of Awakening, around 470 AD, there arose a group who refused to repress their emotions in favour of logic and decided to turn their backs on Vulcan and the teachings of Surak. They named themselves the Romulans and eventually settled on the twin planets known as Romulus and Remus. The descendants of those who chose to "march beneath the Raptor's wings" eventually formed the Romulan Star Empire.
Romulan Society. Defending the Empire and their own personal honor are of the highest importance to a citizen of the Romulan Empire. Romulans are often seen as being xenophobic toward other species, almost racist, and have been known to isolate themselves for long periods of time. Military and political rank directly influence the Romulan society. While the Senate controls the government, the Tal Shiar, or Romulan secret police, seem to be charged with guarding the security of the Empire and keeping its population in line. Around or just before 2368, a grassroots movement, spearheaded by none other than Ambassador Spock, aimed toward reunification of the Vulcans and Romulans. The Romulan Star Empire formed a brief alliance with the Federation and its allies in the Dominion War.