The Romulam Star Empire are an offshoot of the Vulcan race. They broke off from the Vulcans over 2000 years before, and settled on the twin planets of Romulus and Remus. The Romulans are different from the Vulcans now in two main ways: one, they chose to use their emotions and not to supress them, like the Vulcans, and two, because they have cloaking technology on their ships. This technology lets them fly throughout space unseen and undetected from enemies. It is also great for stealth, and spying, for which the Romulans have gained fame for their excellent secret service, the Tal Shiar. They like to stay unknown for the most part, hence the phrase, "To be unknown, is to be unconquerable". The head of the Romulan Empire is the Praetor, or the leader. However, he does not have authority over everything. The Tal Shiar are the big threat.
The Romulans have been involved in very few wars. The only major one is the Dominion War. They joined the Federation alliance.