The New York Times has a great graphic you should bookmark for watching the returns tonight.
It looks at 512 different possible outcomes in the presidential race: 431 mean a victory for Barack Obama, 43 mean a victory for Mitt Romney, and 5 result in a tie.
What happens if there's a tie? The U.S. House of Representatives picks the president and the Senate picks the vice president.
Has that ever happened? Not exactly. The Electoral College tied in 1800 between Thomas Jefferson and Aaron Burr, who technically were running on the same ticket, but that was under the original system the Founding Fathers set up in which the president was the person with the most EC votes and the vice president the person with the second most, regardless of party. The 12th Amendment changed that in 1803, and there hasn't been a tie since then.