When US citizens vote in presidential elections they are actually voting for a group of officials who make up the Electoral College. These people are known as electors and their job is to choose the president and vice-president of the United States.
Each state gets as many electors as it has lawmakers in the US Congress (The Senate and the House of Representatives). California has the most electors (55), while a handful of sparsely populated states like Wyoming, Alaska and North Dakota (and Washington DC) have the minimum of three. There are 538 electors in total. Each elector represents one electoral vote, and a candidate needs to gain a majority of the votes - 270 or more - to win the presidency.
Source: National Archives
Video: CNN: The Electoral College and Your Vote
Although the majority of candidates who have won the popular vote have also won the Electoral College, five presidents have been elected who did not receive the most popular votes. The most recent was Republican Donald Trump, who won the 2016 election although his opponent, Democrat Hillary Clinton, won the popular vote by nearly 3 million votes.
Click on the links below for more information on the Electoral College: