Nero was a Romulan miner who commanded the mining vessel Narada in the 24th century. Following the destruction of Romulus in 2387, Nero began a campaign of revenge against those he felt responsible for the loss of his home planet. During his travels, he was transported back in time to 2233, where he caused the destruction of the USS Kelvin, an act which resulted in the creation of the alternate reality.
In this alternate reality, Nero was responsible for the destruction of Vulcan and many Starfleet vessels. He attempted to destroy Earth but was stopped by James T. Kirk when the USS Enterprise destroyed the Narada. (Star Trek)