The Romulans were a humanoid species and off-shoot of the Vulcans. After rejecting the teachings of Surak they left Vulcan and eventually settled on the world Romulus. The Romulans were governed by the Romulan Star Empire that rose to be one of the major powers in its region of the Milky Way.
Due to their common ancestry the Romulans share many of the same genetic traits as Vulcans. Both have the notable external feature of pointed elfish ears and upturned eyebrows. They also both have copper based blood that appears green. Some Romulans also have a "V" shaped forehead ridge, while others are externally indistinguishable from their Vulcan cousins. (Star Trek: The Next Generation; Star Trek: The Original Series)
Romulans are not completely similar to their Vulcan cousins. There exists enough differentiation between the two that medical practices known to work on Vulcans will not necessarily work with Romulans. (TNG: "The Enemy")
Romulans do not possess the same mental abilities as Vulcans and lack the emotional control of their distant cousins.
The Romulan people were born out of a split on Vulcan between those who followed the teachings of Surak in relation to emotions and logic and those that did not. Those that rejected Surak's teachings left their homeworld and journeyed the stars, settling on various planets. These travellers eventually came across the planet that would be latter known as Romulus, settling it as their new homeworld and forming into the Romulan Star Empire.
The Romulans developed into a culture that appears paranoid and isolationist. They see themselves as superior to other forms of life making them appear to be arrogant to other species.