In June of this year, Frank Lautenberg, the five term senator from New Jersey, died of natural causes. Governor Chris Christie, a Republican, appointed New Jersey's Attorney General, Jefferey Chiesa, to fill his seat, and called a special election for October the 16th to determine who would fill out the rest of the term. New Jersey, along with Virginia, is one of two states that holds is gubernatorial election one year after presidential elections, but Governor Christie, perhaps for political reasons, chose not to hold the special senatorial election at the same time as the gubernatorial election.
Cory Booker, the young mayor of Newark had built up a bit of fame, often through doing things not specifically related to his political positions. He saved a woman from a house fire, shoveled a constituents driveway during a snow storm, and invited people displaced by Hurricane Sandy to stay in his house. He declared as a candidate for the position. The Republican candidate, Steve Lonegan, was also a mayor, of the city of Bogota, although he was less well known than Booker.
New Jersey's politics have been moving steadily leftwards for two decades, and most people assumed Booker would win easily. He did win, although his margin was smaller than most people would assume: 718,000 votes to 579,000 votes. This may be because a special-off-off year election would attract a more conservative electorate than the normal electorate in New Jersey. It could also be that Booker took his election as a foregone conclusion, and chose not to spend time campaigning.
The seat is probably Booker's as long as he chooses to keep it, barring a major scandal. However, I imagine that once he is in the Senate and must make a series of either boring or controversial decisions, he may lose some of his allure. There is a big difference between saving women from burning buildings and sitting through confirmation hearings for the Assistant Undersecretary of Commerce.