The Romulans are one of the most prominent villains of Star Trek, one of the few alien races in the series that has remained hostile towards the Federation, having never truly been defeated nor allied, though they did join forces with them in the war against the Dominion.
In the 4th century CE Vulcans "who marched under the raptor's wing" left Vulcan in a group of colony ships. After a century of wandering the survivors reached a pair of worlds which they claimed as their own. Establishing themselves on the new world the exiles became Romulans. The Romulans have a striking resemblence to Vulcans (due to the fact they are a Vulcan subspecies) but embrace a power-hungry philosophy centred on military aggression and imperialism, they have been behind the scenes in many of Star Trek's more prominent conflicts and their animosity with the Federation has lasted generations.
In the mid 22nd century, the Romulans fought a war against Earth and its allies. Hard fought the Romulans were unable to defeat Earth or their allies. Retreating behind a treaty negotiated neutral zone they licked their wounds and built up strength to the point where they could attack again.
A century later the Romulans sent a ship to probe Federation defenses in preparation for a full scale attack on the Federation. The ship was disabled by the Enterprise before they could return home, with the Romulan commander committing suicide before his ship could be captured.
It is worth noting that although the Romulans are one of the few central alien races in Star Trek to have never been allied with the Federation they are far less destructive than antagonists such as the Borg - Romulans also play a large role in recent updates to Star Trek Online, which is set in an alternate-universe from the mainstream Star Trek titles. The Romulan Republic in the game, which is thought to be a "terroristic cell" by the Romulan Empire, however, can choose to ally the Klingon Defense Force or the Federation, depending on the player's choices.