As of 2015, the President of the United States is chosen by a group of individuals collectively called the Electoral College. When citizens vote in a presidential election, they are actually choosing whom they want their state's Electoral College representatives to vote for.
The Electoral College chooses the President by voting based upon the results of their state's presidential polls. Each state is allocated a number of representatives in the College, equal to the number of Congress people the state possesses. Most states have a winner-takes-all system, where the candidate that wins the popular vote in the state is the only candidate its Electoral College representatives are allowed to vote for. As of 2015, Nebraska and Maine are the sole exceptions. They have a proportional representation system. In these states, votes from the Electoral College representatives may be divided between candidates, rather than a single candidate getting all of the votes.